NO-sensing performance of vacancy defective monolayer MoS2 predicted by density function theory

Applied Surface Science 434 (2018) 294–306

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NO-sensing performance of vacancy defective monolayer MoS2 predicted by density function theory Feifei Li b , Changmin Shi a,∗ a b

Institute of Condensed Matter Physics, School of Physics and Electric Engineering, Linyi University, Linyi 276005, China State-owned Assets and Laboratory Management, Linyi University, Linyi 276005, China

a r t i c l e

i n f o

Article history: Received 11 July 2017 Received in revised form 11 October 2017 Accepted 24 October 2017 Available online 28 October 2017 Keywords: Monolayer MoS2 Vacancy defects NO molecule Gas sensing

a b s t r a c t Using density functional theory (DFT), we predict the NO-sensing performance of monolayer MoS2 (MoS2 MLs) with and without MoS3-vacancy/S-vacancy defects. Our theoretical results demonstrate that MoS3and S-vacancy defective MoS2 -MLs show stronger chemisorption and greater electron transfer effects than pure MoS2 -MLs. The charge transfer analysis showed pure and defective MoS2 -MLs all act as donors. Both MoS3-vacancy and S-vacancy defects induce dramatic changes of electronic properties of MoS2 -MLs, which have direct relationship with gas sensing performance. In addition, S-vacancy defect leads to more electrons transfer to NO molecule than MoS3-vacancy defect. The H2 O molecule urges more electrons transfer from MoS3- or S-vacancy defective MoS2-MLs to NO molecule. We believe that this calculation results will provide some information for future experiment. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Molybdenum disulfide (MoS2 ) with hexagonal structure had been extensively used in the field of field-effect transistor (FET), hydrogen production, solar cells, and gas sensor [1–8]. It had attracted considerable interest due to its unique physical and chemical properties in the past few decades [9]. The interest arisen from its high surface-to-volume ratio comparable to graphene, indirect-to-direct band-gap and layer-dependent tunable bandgap [10–12]. For example, the MoS2 -based field-effect transistor (FET) exhibited high room-temperature current on/off ratio up to 108 [13]; MoS2 -based gas sensor exhibited high sensitivity to NO gas with a detection limit down to 0.8 ppm [14]. Recently, monolayer MoS2 (MoS2 -MLs) had been synthesized by mechanical exfoliation (ME) [15], physical vapour deposition (PVD) [16], chemical vapour deposition (CVD) [17,18], atomic layer deposition (ALD) [19] and molecular beam epitaxy (MBE) [20]. However, the vacancy defects were unavoidable during material’s synthesis process, which were usually more reactive than perfect sites [21,22]. Vacancy defects also usually played an important role in tailoring the electric properties of MoS2 -MLs [23]. In 2013, Zhou et al. investigated the intrinsic defects in MoS2 -MLs grown by CVD method [21]. They found that monosulfur vacancy (S-vacancy) and vacancy

∗ Corresponding author. E-mail addresses: scm [email protected], [email protected] (C. Shi). https://doi.org/10.1016/j.apsusc.2017.10.167 0169-4332/© 2017 Elsevier B.V. All rights reserved.

complex of Mo and its nearby three sulfur (MoS3-vacancy) were frequently observed in the sample. Later, Hong et al. synthesized MoS2 -MLs by ME, CVD and PVD method [22]. They found that the most common defects were monosulfur vacancy (S-vacancy) for MoS2 -MLs grown by ME. Detection of toxic gas, which caused environmental and public health problems, was extremely important and critical [24–27]. Experiment results showed that the MoS2 -based FET and films exhibited high sensitivity to NO [14] and NH3 gas [25]. In theory, the interaction between pure MoS2 -MLs and gas molecules, such as CO, CO2 , NH3 , NO, O2 , and etc., had been studied [28–30]. The adsorption properties of NO2 and NH3 molecules on Al-, Si- and P-doped MoS2 -MLs had also been investigated [31]. However, they placed a little focus on vacancy defective MoS2 -MLs. In order to further exploit the possibilities of MoS2 -MLs as gas sensor, it was necessary to accomplish a systematic investigation on the adsorption properties of MoS2 -MLs, especially for vacancy defective MoS2 -MLs. Nitrogen oxides like nitrogen monoxide (NO) and nitrogen dioxide (NO2 ) are typical air pollutants that cause environmental problems [32,33]. Nitrogen monoxide (NO) can causes acid rains, photochemical smog and production of ozone [32]. Besides, NO also affects neuron operation such as transcriptional regulation and ion channel functions, thus causing neurodegenerative diseases [34,35]. The detection and the emission control of nitrogen oxides are crucial means to reduce their noxious effects on environmental and human beings [36,37]. Initial results have revealed that MoS2 layers are extremely sensitive to NOX [14].

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Fig. 1. Top (a) and side (b) view of pure MoS2 -ML. Green and yellow balls represent Mo and S atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

In this work, we report the adsorption properties and gas sensing mechanisms of NO molecule on most common MoS3-vacancy and S-vacancy defective MoS2 -MLs, using density functional theory (DFT) method. The stabilities of NO molecule on defective MoS2 -MLs (including MoS3-vacancy and S-vacancy defects) are investigated according to their adsorption energies. Our calculation results reveal that defective MoS2 -MLs show stronger chemisorption and more transferred electrons than pure MoS2 MLs. MoS3-vacancy and S-vacancy defects can effectively improve the adsorption and gas sensing performance of NO molecule on MoS2 -MLs. The Bader charge analysis shows that the electrons transfer from pure and defective MoS2 -MLs to NO molecule. The transferred electrons lead to appearance of hole-carrier in pure MoS2 -ML, which leads to p-type conductivity behavior of pure MoS2 -MLs. The resistances (RP ) of gas sensors based on pure MoS2 MLs decrease with the increasing of hole-carrier concentration. However, the transferred electrons result in a decrease in the electron-carrier concentration in defective MoS2 -MLs because of their n-type semiconductor characteristics. The resistances (RN and RM ) of gas sensors based on defective MoS2 -MLs increase with the decreasing of electron-carrier concentration. The adsorption properties and gas-sensing mechanisms of pure and defective MoS2 -MLs are investigated in detail. In addition, the effects of H2 O molecule on NO-sensing performance of defective MoS2 -MLs are also studied in this research. 2. Computational details 2.1. Calculation methods Density functional theory (DFT) calculations were employed using the projector augmented wave (PAW) pseudopotential in Vienna ab initio simulation package (VASP) [38,39]. The electron–electron exchange and correlation energy were described by the generalized gradient approximation (GGA) pseudopotential with Perdew-Burke-Ernzerhof (PBE) formation [40]. In order to describe the van der Waals (vdw) interaction, we adopt the DFT-D3 method proposed by Grimme [41]. The plane-wave cutoff energy was set to 400 eV. A vacuum thickness of 15 Å between adjacent monolayers was used to avoid the coupling of the interlayer. The Monkhorst-pack k point meshes of 5 × 5 × 1 were employed for geometry and calculation of density of states [42]. All the structures were fully relaxed using the conjugated-gradient algorithm [43]. The convergence criterion in progress of geometry optimiza-

tion was set to 1.0 × 10−4 eV per atom for energy. All calculations were spin-polarized. The Bader charge analysis was used to analyze the valence electron distribution [44]. In the work, adsorption energy (Eads ) between the NO molecule and MoS2 -MLs was defined as [45]: Eads = Esubstrate + Eadsorbate − Esubstrate-adsorbate Where Esubstrate-adsorbate was the total energy of adsorbatesubstrate system in the equilibrium state, Esubstrate and Eadsorbate were the total energies of substrate and adsorbate, respectively. By this definition, a position value, corresponding to an exothermic process, indicated stable adsorption. In addition, the net electrontransfer (Q) between the NO molecule and MoS2 -MLs was defined as: Q = Q ads-NO − Q iso-NO Where Qads-NO and Qiso-NO were the valence electrons of NO molecule in adsorbed state and isolated state, respectively. By this definition, a positive value indicates that the electrons transfer from NO molecule to MoS2 -MLs. 2.2. Calculation models MoS2 with space group P63/mmc was chosen as our object. Its crystal structure parameters come from experiment [46]. The lattice constants were a = b = 3.168 Å, c = 12.322 Å and ␣ = ␤ = 90◦ , ␥ = 120◦ . After optimization, (0 0 1) surface was cleaved off. Subsequently, a 5 × 5 × 1 super-cell along a–c directions with 25 Mo atoms and 50 S atoms was built and a 15 Å vacuum layer was added to the super-cell. Fig. 1 displays the top and side view of pure MoS2 -MLs. The calculated distances of Mo S bonds, Mo Mo bonds, and S S bonds in monolayer MoS2 are 2.41 Å, 3.19 Å and 3.17 Å, respectively. These computational bond lengths are consistent with previous report [37]. 3. Results and discussion 3.1. Results of relaxed defective MoS2 -MLs In this work, we investigate two types of vacancies, sulfur vacancy (S-vacancy) and vacancy complex of Mo and its nearby three sulfur vacancy (MoS3-vacancy). After fully relaxed, the structures with two vacancies are shown in Fig. 2. From Fig. 2a, we can see that the MoS3-vacancy induces structural reconstruction.

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Fig. 2. Top and side view of MoS2 -ML with (a) MoS3 and (b) S vacancy. Green and yellow balls represent Mo and S atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Total density of states (DOSs) of (a) pure MoS2 -ML, (b) MoS2 -ML with MoS3 vacancy and (c) MoS2 -ML with S vacancy.

The S atoms surrounding MoS3-vacancy have obvious displacements toward to the vacancy site. The Mo S bond lengths near MoS3-vacancy diminish from 2.409 Å to ∼2.275 Å. Different from MoS3-vacancy, the neighboring atoms towards S-vacancy site have slightly displacements for S-vacancy. The Mo S bond lengths near S-vacancy diminish from 2.409 Å to ∼2.375 Å, which is consistent with the previous results [47].

In order to investigate the effect of vacancy defects on the electronic structure of MoS2 -MLs, we calculate the density of states (DOSs) of pure and defective MoS2 -MLs, as shown in Fig. 3. For pure MoS2 -ML, the Mo atom is in +4 state and the O atom is in −2 state. The bonding form proves the absence of unpaired electrons in pure MoS2 -ML, which play a crucial role in magnetism. As shown in Fig. 3a, we can see that no spin-polarization emerges

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Table 2 Calculated adsorption parameters of NO molecule in their relaxed stable adsorption configurations. Model 1

N N2 N3 N4 N5 N6 N7

Fig. 4. The three different adsorption structures of NO molecule on pure MoS2 -ML. Green, yellow, blue and red balls represent Mo, S, N and O atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 1 The calculated adsorption parameters of NO molecule in their relaxed stable adsorption structures. Model

D (Å)

DN-O (Å)

␤ (◦ )

Q (e)

Eads (eV)

P1 P2 P3

2.77 2.98 2.87

1.17 1.18 1.17

114.02 163.23 109.92

0.04 0.01 0.03

−0.04 0.01 0.14

around the Fermi energy level, which is just crossing the forbidden energy band. The pure MoS2 -ML is non-magnetic, which is consistent with previous calculation reports [23]. The appearance of MoS3-vacancy and S-vacancy defects all induces 2e electrons into defective MoS2 -MLs. The valence electrons of defective MoS2 -MLs are redistributed. After redistribution, the additional 2e electrons are in opposite spin states. Therefore, the defective MoS2 -MLs are non-magnetic, as shown in Fig. 3b and c. The energy states of MoS3vacancy and S-vacancy defects appear at the range of ∼0.73 eV to ∼1.20 eV. The theoretical results of PDOSs reveal that the energy states of vacancy defects are mainly root from Mo atom. 3.2. Adsorption of NO molecule on pure MoS2 -MLs To investigate the adsorption of NO molecule on pure MoS2 -MLs, three types of initial adsorption structures are considered: (P1 ) NO molecule perpendicular to pure MoS2 -ML with N atom down; (P2 ) NO molecule perpendicular to pure MoS2 -ML with O atom down; (P3 ) NO molecule parallel to pure MoS2 -ML. The three initial and relaxed adsorption structures are displayed in Fig. 4. Table 1 summarizes the calculated results of their adsorption structures. D is the nearest distance between adsorbed NO molecule and pure MoS2 MLs; DN-O is the N O bond length of adsorbed NO molecule; ␤ is the angle of ∠O-N-S/∠N-O-S; Q is the transferred electrons between NO molecule and pure MoS2 -MLs and Eads is the adsorption energy. As shown in Table 1, the nearest distances between adsorbed NO molecule and pure MoS2 -MLs are 2.77 Å, 2.98 Å and 2.87 Å for adsorption structures P1 , P2 and P3 . The adsorption energies for NO molecule adsorbed on pure MoS2 -MLs are −0.04 eV, 0.01 eV and 0.14 eV for P1 , P2 and P3 modes. The adsorption energies and distances indicate that the chemisorption of NO molecule on pure MoS2 -MLs is weak. By our definition, Eads > 0, corresponding to an

D (Å)

DN-O (Å)

␤ (◦ )

Q (e)

Eads (eV)

2.66 3.09 2.91 2.11/2.12 2.29/2.28 3.21 3.26

1.17 1.17 1.17 1.22 1.25 1.17 1.17

114.84 120.79 114.26 – – 130.99 112.27

0.01 −0.02 −0.01 0.52 0.42 0.08 0.03

0.15 0.09 0.01 1.95 0.35 0.47 0.24

exothermic process, indicates a more stable structure. We know that the relaxed adsorption structure P3 is the most stable one, duo to its largest adsorption energy. For the most stable adsorption structures P3 , the NO molecule acts as an acceptor and captures 0.03e electrons from pure MoS2 -ML according to the Bader charge analysis. The result is consistent with previous reports [28]. The transferred electrons lead to appearance of hole-carrier in pure MoS2 -ML. Therefore, after NO molecule adsorbs on, the resistance (RP ) of gas sensors based on pure MoS2 -ML decreases with the increasing of hole-carrier concentration duo to its p-type conductivity behavior. 3.3. Adsorption of NO molecule on MoS3-vacancy defective MoS2 -MLs Similarly, NO molecule is initially placed at various sites of MoS3-vacancy defective MoS2 -MLs. Seven initial adsorption configurations are considered: (N1 ) NO molecule perpendicular to MoS2 -ML on the top of S atom around MoS3-vacancy with N atom down; (N2 ) NO molecule perpendicular to MoS2 -ML on the top of S atom around MoS3-vacancy with O atom down; (N3 ) NO parallel to MoS2 -ML around MoS3-vacancy; (N4 ) NO molecule perpendicular to MoS2 -ML in VS site of MoS3-vacancy with N atom down; (N5 ) NO molecule perpendicular to MoS2 -ML in VS site of MoS3-vacancy with O atom down. (N6 ) NO molecule perpendicular to MoS2 -ML in VMo site of MoS3-vacancy with N atom down; (N7 ) NO molecule perpendicular to MoS2 -ML in VMo site of MoS3-vacancy with O atom down. Fig. 5 displays the seven different initial adsorption structures and corresponding relaxed structures. Table 2 summarizes the calculated results of the relaxed configurations. D is the nearest distance between adsorbed NO molecule and defective MoS2 -MLs; DN-O is the N O bond length; ␤ is the angle of ∠ON-S/∠N-O-S; Q is the electron transfer between NO molecule and defective MoS2 -MLs and Eads is the adsorption energy. From Table 2, we can see that the nearest distances (D) are 2.66 Å, 3.09 Å, 2.91 Å, 3.21 Å and 3.26 Å for N1 , N2 , N3 , N6 and N7 , respectively. The N O bond lengths of NO molecule are all 1.17 Å, which is consistent with the free NO molecule [29,48], for N1 , N2 , N3 , N6 and N7 . The nearest distances and N O bond lengths indicate a weak chemisorption between NO molecule and MoS3-vacancy defective MoS2 -MLs with N1 , N2 , N3 , N6 and N7 configurations. For N4 and N5 modes, NO molecule locates at VS site with N atom down and O atom down, respectively. The new formed Mo N bond lengths are 2.11 Å and 2.12 Å for N4 mode, and the new formed Mo O bond lengths are 2.29 Å and 2.28 Å for N5 mode. For N4 and N5 modes, the absorbed N O bond lengths of NO molecule are 1.22 Å and 1.25 Å, which indicates an activation of N O bond upon adsorption. From Table 2, we can see that N4 has the largest adsorption energy, which corresponds to the most stable adsorption structure. In this adsorption structure, 0.52e electrons transfer from MoS3-vacancy defective MoS2 -MLs to NO molecule based on the Bader charge analysis. The MoS3-vacancy defective MoS2 -MLs exhibit n-type conductivity behaviors duo to the additional electrons induced by MoS3-vacancy. So when NO molecule adsorbed

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Fig. 5. The seven different adsorption structures of NO molecule on MoS2 -ML with MoS3 vacancy. Green, yellow, blue and red balls represent Mo, S, N and O atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 3 The calculated adsorption parameters of NO molecule in their relaxed stable adsorption structures. Model 1

M M2 M3 M4 M5

D (Å)

DN-O (Å)

␤ (◦ )

Q (e)

Eads (eV)

2.69 3.10 2.82 2.24/2.14/2.16 2.28/2.31/2.25

1.17 1.17 1.24 1.26 1.29

115.62 108.64 110.10 – –

0.00 0.00 0.07 0.77 0.66

0.13 1.34 0.46 2.57 0.11

on MoS3-vacancy defective MoS2 -ML, the resistance (RN ) of gas sensors based on MoS3-vacancy defective MoS2 -ML materials will increase with the decreasing of electron-carrier concentration. The results are consistent with the pervious experiment reports [14]. 3.4. Adsorption of NO molecule on S-vacancy defective MoS2 -MLs A free NO molecule is introduced into S-vacancy defective MoS2 MLs to simulate the adsorption process. Five initial adsorption configurations are considered: (M1 ) NO molecule perpendicular to defective MoS2 -MLs on the top of S atom around VS with N atom down; (M2 ) NO molecule perpendicular to defective MoS2 ML on the top of S atom around VS with O atom down; (M3 ) NO molecule parallel to defective MoS2 -ML around VS ; (M4 ) NO molecule perpendicular to defective MoS2 -ML in S-vacancy with N atom down; (M5 ) NO molecule perpendicular to defective MoS2 -ML in S-vacancy with O atom down. The five different initial adsorption configurations and their relaxed structures are shown in Fig. 6. Table 3 summarizes the calculated results of adsorption structures. D is the nearest distance between adsorbed NO molecule and defective MoS2 -ML; DN-O is the bond length of adsorbed NO molecule; ␤ is the angle of ∠O-N-S/∠N-O-S; Q is the electron transfer between NO molecule and defective MoS2 -ML and Eads is their adsorption energy. From Table 3, it can be see that the values of nearest distances (D) are 2.69 Å, 3.10 Å and 2.82 Å for M1 , M2 and M3 modes, which indicates weak chemisorption. After relaxed, NO molecule locates at the vacancy site with N atom down, for M4 mode. The new formed Mo N bond lengths are 2.24 Å, 2.14 Å and 2.16 Å, which is consistent with previous results [49]. For M5 mode, the NO molecule locates at the vacancy site with O atom down. The new formed Mo O bond lengths are 2.28 Å, 2.31 Å and 2.25 Å, respectively.

In addition, the N O bond lengths of adsorbed NO molecule are larger to 1.26 Å for M4 mode and 1.29 Å for M5 mode, indicating an activation of N O bond upon adsorption. The adsorption energies Eads are 2.57 eV and 0.11 eV for M4 and M5 modes, indicating M4 is the most stable adsorption structure. The calculated results of Q imply that the electrons transfer from S-vacancy defective MoS2 -MLs to NO molecule. The number of electron transfer is 0.77e electrons for the most stable adsorption structure M4 according to the Bader charge analysis. In the previous calculation reports, Mo vacancies bring about acceptor-like levels and p-type conductivities, whereas S vacancies lead to donor-like levels and n-type conductivities [50]. So when NO molecule adsorbed on S-vacancy defective MoS2 -MLs, the resistance (RM ) of gas sensors based on S-vacancy defective MoS2 -MLs will increase with the decreasing of electron-carrier concentration. The results are also consistent with pervious experiment [14].

3.5. Analysis results In order to have a clearly understanding about the bonding mechanism of NO molecule on pure and defective MoS2 -MLs (including MoS3-vacancy and S-vacancy), we analyzed the corresponding density of states (DOSs) for free and adsorbed NO molecule in adsorption structures P3 , N4 and M4 , as shown in Fig. 7. The molecular orbitals of free NO molecule are labeled as 4␴*, 1␲, 5␴ and 2␲*, with spin-up states (subscript u) and spindown states (subscript d), shown in Fig. 7a. Compared with the free NO molecule, we can see that the 2␲* orbital is significant change and split into several peaks after NO molecule adsorbed on pure or defective MoS2 -MLs, suggesting P3 , N4 and M4 models are chemisorption. The density of states (DOSs) of adsorbed NO molecule change greatly near the Fermi energy level (EF ) in P3 , N4 and M4 modes, especially for 2␲* orbitals. For 2␲u * and 2␲d * energy states, parts of energy states are pushed below EF and broaden to a wider energy state from ∼−0.25 eV to 0 eV for P3 mode and ∼−6.5 eV to 0 eV for N4 and M4 modes; parts of energy states are pushed above EF and also broaden to a wider energy state from 0 eV to ∼2.34 eV for P3 model, ∼4.52 eV for N4 model and ∼4.70 eV for M4 model. The emerging energy states in Fig. 7b–d indicate that some electrons break away from NO molecule and some electrons transfer to NO molecule. The broadening of 2␲* orbitals are primarily the results of 2␲* donation and back-donation [51,52]. In total,

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Fig. 6. The five different adsorption structures of NO molecule on MoS2 -ML with S vacancy. Green, yellow, blue and red balls represent Mo, S, N and O atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Total density of states (DOSs) of (a) free NO molecule, (b) adsorbed NO on pure MoS2 -ML, (c) adsorbed NO on MoS2 -ML with MoS3 vacancy and (d) adsorbed NO on MoS2 -ML with S vacancy. The vertical dotted line indicates the Fermi energy level.

2␲* orbitals exhibit back-donation based on Bader charge analysis. We can estimate the amount of back-donated electronic charge by integrating over the squared amplitude of the 2␲* orbital located below EF [53]. The DOSs indicate the defective MoS2 -MLs have electrons transfer to NO molecule, which is consistent with the Bader charge analysis. All these suggest that there is chemisorption and significant electron transfer between adsorbed NO molecule and pure/defective MoS2 -MLs. For pure MoS2 -MLs, the NO molecule

obtains 0.03e electrons in the most stable adsorption structure P3 . The transferred electrons lead to appearance of hole-carrier. The intrinsic semiconductor (pure MoS2 -MLs) transforms to p-type semiconductor, which would exhibit p-type conductivity behaviors. The appearance of hole-carrier in pure MoS2 -MLs leads to a decrease of resistance (RP ) for gas sensors based on pure MoS2 -MLs. For MoS3-vacancy defective MoS2 -MLs, the MoS3-vacancy induces 2e electrons into defective MoS2 -MLs. The MoS3-vacancy results in the intrinsic semiconductor transforms to n-type semiconductor, which would exhibit n-type conductivity behaviors. In this case, the NO molecule obtains 0.52e electrons in most stable adsorption structure N4 . The transferred electrons lead to a decrease of electron-carrier concentration, and then lead to an increase of resistance (RN ) of gas sensors based on MoS3-vacancy defective MoS2 -MLs materials. For S-vacancy defective MoS2 -MLs, the Svacancy also induces 2e electrons into defective MoS2 -MLs. After NO molecule adsorbed on S-vacancy defective MoS2 -MLs, the NO molecule obtains 0.77e electrons in most stable adsorption structure M4 . The calculated results lead to a decrease in electron-carrier concentration and an increase in resistance (RM ) of gas sensors based on S-vacancy defective MoS2 -MLs. Compare with N4 and M4 modes, it is evident that the S-vacancy defect induces more significant electron transfer effects than MoS3-vacancy defect between NO molecule and defective MoS2 -MLs (from 0.52e to 0.77e). The electrons transferred from defective MoS2 -MLs to NO molecule will affect the electronic properties of defective MoS2 -MLs. More electrons transferred to NO molecule will lead to larger decrease in electron-carrier concentration and larger increase in resistance, which indicates a better adsorption or gas-sensing performance for gas sensors based on defective MoS2 -MLs materials. The theoretical results are consistent with the experiment results when NO molecule adsorbed on MoS2 -MLs materials [14]. According to the above mentioned, we know that the size order of electron transfer (Q) is: QP < QN < QM , where QP , QN , QM represent the electron transfer in adsorption structures P3 , N4 and M4 . The size order of resistances (R) of gas sensors is: RP  RN ≈ RM , where RP , RN , RM represent the resistances of gas sensors based on pure MoS2 -ML, MoS3-vacancy defective MoS2 -ML and S-vacancy defective MoS2 -ML, respectively. In experiment, the gas sensing response is usually defined as: ␩ = (Rg − Ra )/Ra = R/Ra , where Rg is the resistance measured under a working circumstance, while Ra is the resistance in air. The R is in direct proportion to the electron transfer Q, and the gas sensing response ␩ is in direct proportion to Q/Ra . Therefore, the size order of gas sensing response of gas sensors is: ␩P < ␩N < ␩M , where ␩P , ␩N , ␩M represent the gas sensing response of gas sensors based on pure MoS2 -ML, MoS3-

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Fig. 8. Total density of states (DOSs) of (a)(d) pure MoS2 -ML after NO molecule adsorbed on, (b)(e) MoS3 vacancy defective MoS2 -ML after NO molecule adsorbed on and (c)(f) S vacancy defective MoS2 -ML after NO molecule adsorbed on; The vertical dotted line indicates the Fermi energy level.

vacancy defective MoS2 -ML and S-vacancy defective MoS2 -ML, respectively. So, in practical application, we can improve the gas sensing response of gas sensors by increasing the electron transfer, making the S-vacancy defects as much as possible. The electron transfer can be modulated by vacancy defect types and vacancy defect concentration. The adsorption energy for M4 mode (2.57 eV) is much bigger than N4 mode (1.95 eV) and P3 mode (0.14 eV). It means that the adsorption structure becomes more stable with S-vacancy in MoS2 MLs. The total DOSs of pure and defective MoS2 -MLs (including MoS3-vacancy and S-vacancy) with NO molecule are shown in Fig. 8. The DOSs of MoS2 -MLs in adsorption structures P3 , M4 and N4 change greatly and exhibit spin-splitting around the Fermi energy (Ef ). The DOSs reveal that the magnetic moments are all ∼1.0 ␮B for P3 , N4 and M4 modes after NO molecules adsorb on. The magnetic moments are caused by the asymmetry of structures and electrons. From Fig. 8b, we can see that two new local energy levels occur near Fermi level, suggesting that the adsorption of NO molecule can change electronic properties of MoS3-vacancy defective MoS2 MLs. Compared Figs. 3 c and 8 c, we can see that the vacancy defect states above Femi energy disappear after the NO adsorbed on, due to the fix of N atom on the S-vacancy, which is similar to the previous report [54]. However, two new local energy levels occur near Fermi level, suggesting that the adsorption of NO molecule can also change electronic properties of S-vacancy defective MoS2 -MLs. Fig. 9 displays the partial density of states (PDOSs) of N atom of NO molecule interacts with the surface Mo atom of MoS2 -MLs after NO molecule adsorbed on with M4 and N4 modes. From Fig. 9a and b, we can see that large overlap between NNO -2p and MoMoS2 -3d orbitals is evident for M4 and N4 modes. Such results imply the formation of new Mo N bonds and there is strong chemisorption between NNO atoms and MoMoS2 atoms. Our calculation results show that the two types of vacancy defects in MoS2 -MLs can significantly improve the bonding energy between the adsorbed NO molecule and MoS2 -MLs. Compared with pure MoS2 -ML, the defective MoS2 -ML (including MoS3-vacancy and S-vacancy) are more suitable for NO detection. Fig. 10 represents the isosurfaces of HOMOs and LUMOs for NO molecule adsorbed on defective MoS2 -MLs with M4 and N4 modes, respectively. As shown in Fig. 10a and b, the electrons of HOMO and LUMO are localized around NO molecule, new formed Mo N bonds and the surrounding Mo atoms. Different from MoS3-

Fig. 9. Partial density of states (PDOSs) of N-2p (from NO) and Mo-3d (from MoS2 ) in adsorption structures (a) M4 mode and (b) N4 mode. The vertical dotted line indicates the Fermi energy level.

vacancy defective MoS2 -MLs (shown in Fig. 10a and b), it can be seen from Fig. 10c and d, the electrons of HOMO are mainly dominant on the NO molecules and the new formed Mo N bonds, whereas the electrons of LUMO are dominant on Mo atoms of Svacancy defective MoS2 -MLs. The accumulation of the electronic density at the middle of the newly formed bonds confirms the formation of the new bonds and consequently the transfer of electronic density from the Mo N bonds and N O bonds to the newly formed bands [37]. To understand the bonding of NO molecule on defective MoS2 -MLs surface, electron density difference [␦r = ␳(MoS2+NO) − ␳(MoS2) − ␳(NO) ] is calculated, which illustrates

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Fig. 10. The (a)/(c) HOMO and (b)/(d) LUMO of NO molecule on MoS2 -MLs with (a)/(b) MoS3-vacancy and (b)/(d) S-vacancy defects in M4 mode and N4 mode, respectively.

Fig. 11. Isosurfaces of difference electron densities [␦␳ = ␳(MoS2+NO) − ␳(MoS2) − ␳(NO) ] of NO molecule on (a) MoS3-vacancy and (b) S-vacancy defective MoS2 -MLs in M4 mode and N4 mode, respectively. Red isosurfaces represent ␦␳ > 0 (excess) and green ones correspond to ␦␳ < 0 (depletion). (isovalue = 0.003 e/Å3 ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

how the electron density changes during this process. Fig. 11a and b shows the isosurfaces of differential electron density of NO molecule adsorbed on MoS2 -MLs with MoS3 vacancy and S vacancy defects, respectively. In the figures, the red and green regions represent the areas of electron accumulation and electrons loss, respectively. From Fig. 11a and b, it is clear that the regions of charge depletion and accumulation located on the NO molecule and the new formed Mo N bonds, which indicating large amount of electron transfer. Fig. 11a and b indicate that the electrons accumulate on NO molecule and the new formed Mo N bonds. Combine with Bader analysis the accumulation of electrons on NO molecule are 0.52e and 0.77e for defective MoS2 -MLs with M4 and N4 modes, respectively. 3.6. Effect of H2 O molecule on NO-sensing performance on MoS2 -MLs Water is the most abundant compound in the biosphere and covers most real solid surface [55]. The adsorption of water has important consequences for surface process such as catalysis and gas sensing [56]. To investigate the effects of H2 O molecule on NOsensing on MoS2 -MLs, H2 O molecule is initially placed at various

sites of monolayer MoS2 . For H2 O molecule on pure MoS2 -MLs, previous reports have proved that it belongs to physisorption [28]. For MoS3-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule, four types of initial adsorption structures are considered: (NH 1 ) H2 O molecule perpendicular to defective MoS2 -MLs on the top of O atom with H atom down; (NH 2 ) H2 O molecule perpendicular to defective MoS2 -ML on the top of S atom with H atom down; (NH 3 ) H2 O perpendicular to defective MoS2 -ML on the top of S atom with O atom down; (NH 4 ) H2 O molecule perpendicular to defective MoS2 -ML around MoS3-vacancy. Fig. 12 displays the four different initial adsorption structures and corresponding relaxed structures. Table 4 summarizes the calculated results of the relaxed configurations. D is the nearest distance between adsorbed H2 O molecule and MoS3-vacancy defective MoS2 -ML with preadsorbed NO molecule; DMo-N is the Mo N bond length; DN-O is the N O bond length; DH-O is the H O bond length; ␤ is the angle of ∠O-H-O; QNO is the electron transfer between NO molecule and MoS3-vacancy defective MoS2 -MLs; QH2O is the electron transfer between H2 O molecule and MoS3-vacancy defective MoS2 -MLs and Eads is the adsorption energy. As showed in Table 4, the nearest distances between adsorbed H2 O molecule and MoS3-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule are 2.05 Å,

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Fig. 12. The four different adsorption structures of H2 O molecule on MoS3-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule. Green, yellow, blue, red and white balls represent Mo, S, N, O and H atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 4 The calculated adsorption parameters of NO and H2 O molecules in their relaxed stable adsorption structures. Model

D (Å)

DMo-N (Å)

DN-O (Å)

DH-O (Å)

␤ (◦ )

QNO (e)

QH2O (e)

Eads (eV)

NH 1 NH 2 NH 3 NH 4

2.05 2.32 2.93 2.18

2.10/2.10 2.11/2.12 2.08/2.14 1.98/2.08

1.23 1.22 1.22 1.35

0.97/0.97 0.98/0.97 0.97/0.97 0.98/1.65

105.23 104.40 105.58 102.64

0.58 0.51 0.53 1.09

−0.01 0.01 −0.02 −0.16

0.18 0.15 0.06 1.31

2.32 Å and 2.93 Å for adsorption structures NH 1 , NH 2 and NH 3 . For NH 4 mode, the H2 O molecule reacts with MoS3-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule when a H2 O molecule is introduced into. This reaction leads to dissociative adsorption of H2 O molecule: one H␤− atom chemically adsorbs on the preadsorbed NO molecule and the OH␣− group locates at vacancy site with O atom down, as shown in Fig. 13a. The reaction can be described as: ␤−

H2 O(gas) + Vn− + NOm− = OH␣− + NOH(ads) (ads) (ads) where “gas” and “ads” represent water in gas state and NOm− /OH˛− /NOHˇ− group in adsorbed state, respectively. The new formed Mo O bond lengths are 2.18 Å and 2.19 Å, and the new formed NO H bond length is 1.02 Å for NH 4 mode. For NH 4 mode, the optimized nearest distance (D) is 2.18 Å, while the optimized bond lengths of dissociated H2 O molecule (dHO-H ) is 1.65 Å. The adsorption energies for H2 O molecule adsorbed on MoS3-vacancy defective MoS2 -MLs with NO molecule are 0.18 eV, 0.15 eV, 0.06 eV

and 1.31 eV for NH 1 , NH 2 , NH 3 and NH 4 modes. From Table 4, we can see that NH 4 has the largest adsorption energy, which corresponds to the most stable adsorption structure. In this adsorption structure, 1.09e electrons transfer from MoS3-vacancy defective MoS2 -MLs to NO molecule and 0.16e electrons transfer from H2 O molecule to MoS3-vacancy defective MoS2 -MLs based on the Bader charge analysis. The H2 O molecule urges more electrons transfer from MoS3-vacancy defective MoS2 -MLs to NO molecule (from 0.52e to 1.09e). The MoS3-vacancy defective MoS2 -MLs lose 0.93e electrons in total (including NO and H2 O molecules). So when NO and H2 O molecules simultaneously adsorbed on MoS3-vacancy defective MoS2 -MLs, there will have larger decrease of electroncarrier concentration in MoS3-vacancy defective MoS2 -MLs, and then lead to larger increase of resistance (RN ) of gas sensors based on MoS3-vacancy defective MoS2 -MLs materials, which indicate better gas-sensing performance. To understand the bonding of H2 O molecule on MoS3-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule, electron density difference [␦␳ = ␳(MoS2+NO+H2O) − ␳(MoS2) − ␳(NO) − ␳(H2O) ] is

F. Li, C. Shi / Applied Surface Science 434 (2018) 294–306

303

Fig. 13. The (a) adsorption structure, (b) difference electron densities [␦␳ = ␳(MoS2+NO+H2O) − ␳(MoS2) − ␳(NO) − ␳(H2O) ], (c) HOMO and (d) LUMO of H2 O molecule on MoS3vacancy defective MoS2 -MLs with pre-adsorbed NO molecule in NH 4 mode. Red isosurfaces represent ␦␳ > 0 (excess) and green ones correspond to ␦␳ < 0 (depletion). (isovalue = 0.003 e/Å3 ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

calculated, which illustrates how the electron density changes during this process. Fig. 13b shows the isosurfaces of differential electron density when H2 O and NO molecules simultaneously adsorbed on MoS3-vacancy defective MoS2 -MLs. In the figures, the red and green regions represent the areas of electron accumulation and electrons loss, respectively. From Fig. 13b, it is clear that the regions of electron depletion and accumulation located on H2 O molecule, NO molecule and Mo N bonds, which indicating large amount of electron transfer. Fig. 13b indicates that the electrons accumulate on NO molecule and Mo N bonds. Combine with Bader analysis the accumulation of electrons in NO molecule are 1.09e and the depletion of electrons in H2 O molecule are 0.16e for MoS3-vacancy defective MoS2 -MLs with NH 4 mode. Fig. 13c and d represent the isosurfaces of HOMO and LUMO when H2 O and NO molecules adsorbed on MoS3-vacancy defective MoS2 -MLs with NH 4 mode. The electrons of HOMO are localized around the Mo and S atoms surrounding the MoS3-vacancy, whereas the electrons of LUMO are dominant on NO molecule and Mo and S atoms surrounding the MoS3-vacancy. The accumulation of the electronic density at the middle of the new formed bonds confirms the formation of new bonds [37]. Similarly, H2 O molecule is initially placed at various sites of S-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule. Four initial adsorption configurations are considered: (MH 1 ) H2 O molecule perpendicular to MoS2 -ML on the top of O atom with H atom down; (MH 2 ) H2 O molecule perpendicular to MoS2 -ML on the top of S atom with O atom down; (MH 3 ) H2 O perpendicular to MoS2 -ML on the top of S atom with H atom down; (MH 4 ) H2 O molecule perpendicular to MoS2 -ML on the top of O atom with two H atoms down. Fig. 14 displays the four different initial adsorption structures and corresponding relaxed structures. Table 5 summarizes the calculated results of the relaxed configurations. D is the nearest distance between adsorbed H2 O molecule and S-vacancy defective MoS2 -MLs; DMo-N is the Mo N bond length; DN-O is the N O bond length; DH-O is the H O bond lengths; ␤ is the angle

Fig. 14. The four different adsorption structures of H2 O molecule on S-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule. Green, yellow, blue, red and white balls represent Mo, S, N, O and H atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of ∠O-H-O; QNO is the electron transfer between NO molecule and S-vacancy defective MoS2 -MLs; QH2O is the electron transfer between H2 O molecule and S-vacancy defective MoS2 -MLs and Eads is the adsorption energy. As showed in Table 5, the nearest distances between adsorbed H2 O molecule and S-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule are 1.85 Å, 2.90 Å, 2.47 Å and 2.37 Å for adsorption structures MH 1 , MH 2 , MH 3 and

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Table 5 The calculated adsorption parameters of NO and H2 O molecule in their relaxed stable adsorption structures. Model 1

MH MH 2 MH 3 MH 4

D (Å)

DMo-N (Å)

DN-O (Å)

DH-O (Å)

␤ (◦ )

QNO (e)

QH2O (e)

Eads (eV)

1.85 2.90 2.47 2.37

2.22/2.16/2.12 2.23/2.16/2.14 2.23/2.15/2.14 2.22/2.17/2.12

1.27 1.26 1.26 1.26

0.98/0.97 0.97/0.97 0.98/0.97 0.97/0.97

104.53 103.85 104.61 102.57

0.84 0.78 0.76 0.81

−0.01 −0.02 0.00 −0.02

0.17 0.13 0.13 0.16

Fig. 15. The (a) adsorption structure, (b) difference electron densities [␦␳ = ␳(MoS2+NO+H2O) − ␳(MoS2) − ␳(NO) − ␳(H2O) ], (c) HOMO and (d) LUMO of H2 O molecule on Svacancy defective MoS2 -MLs with pre-adsorbed NO molecule in MH 1 mode. Red isosurfaces represent ␦␳ > 0 (excess) and green ones correspond to ␦␳ < 0 (depletion). (isovalue = 0.003 e/Å3 ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

MH 4 . The adsorption energies for H2 O molecule adsorbed on Svacancy defective MoS2 -MLs with pre-adsorbed NO molecule are 0.17 eV, 0.13 eV, 0.13 eV and 0.16 eV for MH 1 , MH 2 , MH 3 and MH 4 modes, respectively. From Table 5, we can see that adsorption structure MH 1 has the largest adsorption energy, which corresponds to the most stable adsorption structure. In this adsorption structure, 0.84e electrons transfer from S-vacancy defective MoS2 -MLs to NO molecule and 0.01e electrons transfer from H2 O molecule to S-vacancy defective MoS2 -MLs based on the Bader charge analysis. The H2 O molecule urges electrons transfer from S-vacancy defective MoS2 -MLs to NO molecule (from 0.77e to 0.84e). The Svacancy defective MoS2 -MLs lose 0.83e electrons in total (including NO and H2 O molecules). So when NO and H2 O molecules simultaneously adsorbed on S-vacancy defective MoS2 -MLs, there will also have larger decrease of electron-carrier concentration in S-vacancy defective MoS2 -MLs, and then lead to larger increase of resistance (RN ) of gas sensors based on S-vacancy defective MoS2 -MLs materials, which indicate better NO-sensing performance. To understand the bonding of H2 O molecule on S-vacancy defective MoS2 -MLs with pre-adsorbed NO molecule, electron density difference [␦␳ = ␳(MoS2+NO+H2O) − ␳(MoS2) − ␳(NO) − ␳(H2O) ] is calculated, which illustrates how the electron density changes during this process. Fig. 15b shows the isosurfaces of differential electron density of H2 O and NO molecules simultaneously adsorbed on S-vacancy defective MoS2 -MLs. In the figures, the red and green regions represent the areas of electron accumulation and electrons

loss, respectively. From Fig. 15b, it is clear that the regions of electron depletion and accumulation located on H2 O molecule, NO molecule and Mo N bonds, which indicate large amount of electron transfer. Fig. 15b indicates that the electrons accumulate on NO molecule and Mo N bonds. Combine with Bader analysis the accumulation of electrons on NO molecule are 0.84e and the depletion of electrons from H2 O molecule are 0.01e for S-vacancy defective MoS2 -MLs with MH 1 mode. Fig. 15c and d represents the isosurfaces of HOMO and LUMO when H2 O and NO molecules simultaneously adsorbed on S-vacancy defective MoS2 -MLs with MH 1 mode. The electrons of HOMO are localized around NO molecule, Mo N bonds and Mo atoms surrounding the S-vacancy, whereas the electrons of LUMO are dominant on surface Mo atoms of S-vacancy defective MoS2 -MLs, as shown in Fig. 15d. The accumulation of the electronic density at the middle of the newly formed bonds confirms the formation of new bonds [37]. 4. Conclusions To understand the performance of monolayer MoS2 (MoS2 -ML) as gas sensor, we analyzed the structural and electronic properties of NO molecule on pure and defective MoS2 -MLs (including MoS3-vacancy and S-vacancy). Our theoretical results demonstrate that pure MoS2 -ML show weak chemisorption and little electron transfer between NO molecules and pure MoS2 -ML. The transferred electrons lead to appearance of hole-carrier and p-type conductiv-

F. Li, C. Shi / Applied Surface Science 434 (2018) 294–306

ity behavior in pure MoS2 -ML. After NO molecule adsorbed on, the resistances of gas sensors based on pure MoS2 -ML decrease with the increasing of hole-carrier concentration. Different from pure MoS2 -ML, MoS3-vacancy and S-vacancy defective MoS2 -MLs show stronger chemisorption and greater electron transfer effects than pure MoS2 -ML. The MoS3-vacancy and S-vacancy defective MoS2 MLs act as donors, electrons transfer to NO molecule from defective MoS2 -ML. The resistances of gas sensors based on defective MoS2 MLs increase with the decrease of electron-carrier concentration for defective MoS2 -MLs. The MoS3-vacancy and S-vacancy defects effectively induce dramatic changes of electronic properties for MoS2 -MLs, which have direct relationship with the gas sensing performance. The S-vacancy defects induce more electrons transfer to NO molecule than MoS3-vacancy defects, implying S-vacancy defects lead to a larger decrease of electron-carrier concentration and a larger increase of resistances of gas sensors based on defective MoS2 -MLs. Our results demonstrate that two types of vacancy defects can effectively improve the NO adsorption performance and gas sensing performance. The effects of H2 O molecule on NO-sensing performance of defective MoS2 -MLs are also studied. The H2 O molecule urges more electrons transfer from defective MoS2 -MLs to NO molecule about 0.57e (from 0.52e to 1.09e) and 0.07e (from 0.77e to 0.84e) for MoS3-vacancy and S-vacancy defects, respectively. In total, the defective MoS2 -MLs lose 0.93e and 0.83e for MoS3-vacancy and S-vacancy defects when H2 O and NO molecules simultaneously adsorbed on. The results lead to larger increase of resistance of gas sensors based on defective MoS2 MLs materials, which indicate better NO-sensing performance.

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Acknowledgements [27]

This study was supported by National Natural Science Foundation of China (11647155), Shandong Colleges Science and Technology Program (J16LJ04) and Linyi University (LYDX2016BS022).

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