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The enhancement of NO detection by doping strategies on monolayer MoS2
Vacuum 130 (2016) 146e153
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The enhancement of NO detection by doping strategies on monolayer MoS2 Kaining Ding*, Yihua Lin, Mengyue Huang Department of Chemistry, Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian, 350108, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 30 April 2016 Accepted 3 May 2016 Available online 4 May 2016
First principles calculations are carried out to investigate the reactivities of pure monolayer MoS2 nanosheet doped with non-metal elements X (X ¼ B, P, Cl) in the presence of NO. All doped systems exhibit strong effects on NO, especially for the B- and P-doped MoS2 cases, whereas the pure MoS2 shows weak physisorption of NO. To fully explore the mechanism of strengthening interactions between the substrate and NO, we discuss the changes in the electronic structure, which determines the electrical conductivity. We suggest that P-doped MoS2 is a suitable candidate for sensing NO polluting gas, while the chemisorption of NO on B-doped MoS2 is so strong that NO desorption is difficult. © 2016 Elsevier Ltd. All rights reserved.
Keywords: MoS2 monolayer Doped system Electronic structure Gas sensor
1. Introduction Few-layered MoS2, which is a graphene-like two-dimensional (2D) layered transition metal dichalcogenide material, has attracted enormous interest for its promising semiconducting characteristics and the advantageous band gap than graphene. Graphene, the most well-known 2D material, has been the focus of extensive attention for a long time, including as gas sensor material [1e3]. However, its zero band gap, which causes over high charge carrier mobility, limits its applications. In contrast, the unique monolayer MoS2 intriguingly shows a desirable and direct band gap of 1.90 eV. And the intrinsic semiconductor nature [4,5] and large surface-tovolume ratio make monolayer MoS2 appropriate in chemical sensors [6,7]. Owing to the distinctive physical, electronic and optical properties, MoS2 has become an important candidate in nanoelectronic and optoelectronic fields like transistors [8], photoemitting devices [9], high-performance catalysis [10,11], dry lubrication [12,13], batteries [14,15] and photovoltaics [16,17]. Toxic NO gas is known to be a highly important air pollutant. A certain concentration of NO can cause serious respiratory and skin problems among people. Therefore, materials able to detect NO are
* Corresponding author. E-mail address: [email protected] (K. Ding). http://dx.doi.org/10.1016/j.vacuum.2016.05.005 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
urgently needed. Multilayer MoS2 films based on transistor sensors have been experimentally demonstrated to show stable sensitivity towards NO gas molecules [6], whereas single-layer MoS2 films exhibit sharp but unstable current responses. For this reason, we are interested in decorating the surface of monolayer MoS2 to improve its chemical sensitivity to NO. Impurity doping may be a desirable approach to modulate the electronic properties of monolayer MoS2. Qin et al. synthesized Ndoped MoS2 nanosheets by the sol-gel approach [18]; Yang et al. experimentally doped 2D WS2 and MoS2 with chlorine molecules and concluded that Cl doping could be applied in the fabrication of high-performance electronic devices [19]. Liu et al. prepared B- and N-doped MoS2 nanosheets as fluorescent probes to detect the heavy metal ion Hg2þ with high sensitivity and selectivity [20]. Yue et al. theoretically substituted non-metal atoms (H, B, C, O, N, and F) and transition metal atoms into MoS2 sheets in VS defects [21] and found that all substituted atoms are favorable bound to VS defects. Various discoveries regarding the surface modification of nanostructured materials have been reported. For example, B- and S-doped graphenes have been shown to chemically bind NO2 [22]. Zhang et al. demonstrated that using B-doped graphene could significantly improve the sensitivity and selectivity of gas sensors [23]. Incorporating Au into SnO2 thick films was found to be a promising method to improve the CO gas response [24] Al-doped SnO2 thin films obtained via the rheotaxial growth and thermal oxidation (RGTO) method also exhibited enhanced gas sensing [25].
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In addition, Si-doped SnO2 films are useful liquid petroleum gas sensors [26]. Defects can significantly change the electronic properties of h-AlN sheets [27]. F-doped corannulene molecules are more stable than H-doped (hydrogenated) ones for potential applications in C-based nanostructures [28]. Few studies have investigated the adsorption of small gas molecules on monolayer MoS2. Yue et al. showed that different molecules, such as NH3, NO, and NO2, only physisorbed to monolayer MoS2 with a small charge transfer, which could be significantly modulated by a perpendicular electric field [29]. Zhao et al. also reported that among small gas molecules, only NO and NO2 exhibited stronger binding to MoS2 sheets [30]. Both of the studies provide a theoretical basis for the potential application of MoS2 monolayers as polluting gas sensors for gases like NO and NO2. However, a systematic investigation into responses obtained by exposing doped and perfect MoS2 nanosheets to NO gas has not been undertaken. Here, to choose the best dopants, we explored NO gas adsorption by doping a series of non-metals (B, C, N, O, F, Si, P and Cl) into MoS2. We found that C-, Si-, N-doped MoS2 had similar adsorption activities to P-doped MoS2, that O and F doping caused almost no change in the NO adsorption, and that the effect of Cl doping was slightly better than those of O and F. The B-doped case showed the strongest adsorption. Hence, we selected B, P, and Cl as dopants according to the observed adsorption strengths. In addition, we expect that B may be the potential candidate like its application in B-graphene based gas sensors. P and Cl atoms are at the same period as S in the periodic table. The covalent radius of B, P, and Cl are approximately equivalent to that of S, and thus, B, P and Cl readily form stable bonds with Mo. Most importantly, their p orbitals contain odd numbers of electrons, and therefore, changes in their electronic densities of states (DOSs; spin-splitting states) can be clearly compared. We used the doped MoS2 to detect NO gas and explored the possibilities of transforming the gas adsorption from physisorption to certain types of chemical adsorption. Although physisorption can create changes in conduction, the effects of chemisorption are relatively visible and thermally stable [22]. By calculating the adsorption energies and exploring the changes in the electronic properties and charge transfer, we discuss the interaction mechanisms between the substrates and NO and their effects on the sensing properties. We expect that these results will constitute a solid foundation for designing gas-sensing devices. 2. Computational method and details Our calculations are carried out by density functional (DFT) theory based on the first-principles and performed by the Cambridge Serial Total Energy Package (CASTEP) code [31]. The generalized gradient approximation of Perdew-Wang 91 gradientcorrected function (GGA-PW91) [32] is used to optimize the geometrical configurations of pure and doped MoS2. Considering the large size effect,we construct the 4 4 supercell for monolayer MoS2 which consists of 32 sulfur atoms and 16 molybdenum atoms. To avoid interlayer interactions, the vacuum thickness was set to 18 Å. After testing, we set the Brillouin zone sampling k-points meshes as 8 8 1 and kinetic cut-off energy for the plane-wave expansion as 440 eV. The valence atomic electrons configurations are 3s23p4 for S, 4d55s1 for Mo, and 2s22p1 for B, 2s22p3 for N, 2s22p4 for O, 3s23p3 for P and 3s23p5 for Cl. The convergence criterion of the energy change, the maximum forces, the stresses on the cell, the atomic displacements were set as 2 105 eV, 0.05 eV/ Å, 0.1 GPa and 2 103 Å. To acquire the appropriate method to explore the electronic structure for these systems, GGA-PW91 [32], GGA-PBE [33], the local density approximation (LDA) [34], the hybrid HSE06 [35]
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function are all employed to calculate the band gap of pure MoS2. Based on the results from different functions, we found that hybrid HSE06 function always gives relatively exact results but excessively overestimate the value of band gap in our MoS2 systems. The remained LDA, GGA-PW91, GGA-PBE method give the similar value for MoS2 band gap. The LDA often overestimated the adsorption energy [29], and the band gap value calculated by GGA-PW91 for 1.72 eV is more close to experimental results (1.90 eV) [9] than that calculated by GGA-PBE for 1.71 eV, the results could reference to Table 1. Therefore, we finally choose the GGA-PW91 method. To prevent the cases of GGA density approximation cannot entirely achieve the weakly bonded van der Waals (vdW) interactions for gas molecule adsorption on monolayer MoS2, we additionally select the OBS method for DFT-D correction. There are also some theoretical studies adopting the OBS method for DFT-D correction to consider the effect of the van der Waals interactions, Ref [36,37]. Compared to conventional DFT calculations, DFT-D method increases the semiempirical pair-wise force field and ensures to correctly account for the effect of vdW interactions. For doping in S vacancy is easier to bind than in Mo vacancy [21], we use single B, P or Cl atom to replace one S atom of MoS2 in TX site, see Fig. 1. The corresponding doping concentration is 2.08%. And the doping formation energies are calculated by the following formula:
Eform ¼ Edoped þ mS EMoS2 mX
(1)
Edoped are total energies of MoS2 doped with B, P, or Cl, and EMoS2 is the total energies for pure MoS2. mx are chemical potentials of B, P and Cl. And mX are taken from their most stable phases, mS ¼ [E(H2S)eE(H2)], mB ¼ 1/2 [E(B2H6)3E(H2)], mP ¼ 1/4 E(P4), mCl ¼ 1/2 E(Cl2). The calculated results are summarized in Table 2. To obtain the most stable optimal adsorption configuration, we considered five adsorption sites. First the TX site on top of the doped site; second, the TM site on top of a Mo atom; third, the TS site on top of a S atom; and fourth, the B site on top of a MoeS bond and the last H site on top of the hollow for a hexagon while in top view. We placed three different gas orientations at each site, one is the NO axis lies parallel to the monolayer. The other two are the NO molecule perpendicular to these sites, with N or O atom pointing downward. And the adsorption energy of gas molecules on MoS2 is defined as:
Eads ¼ ENO=system Esystem þ ENO
(2)
Esystem is the total energy for pure or doped MoS2. And ENO/system represents for the total energy of the system adsorbed with NO. ENO is the total energy of isolated NO molecule. The negative value of Eads indicates the adsorption process is exothermic, likewise, the calculated results are summarized in Table 3.
Table 1 Calculation of single energy and band gap with different methods. Method
GGA/PW91
GGA/PBE
LDA
HSE06
Exp
Eg/eV
1.72 1.70a
1.71 1.67b
1.73 1.87c
2.25 2.21d
1.90e
a b c d e
Ref Ref Ref Ref Ref
[38]. [39]. [40]. [41]. [9].
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Fig. 1. a is the top view of MoS2 monolayer,the marked red rose balls represent for the adsorption site of TX, TM, H and B0 (bond); b, c, d are the side view of B, P, Cl doped MoS2 monolayer in TX site. Each type of atoms has been color coded. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Structural relaxtion parameters, Mulliken charge population (e) on the main atom and bonds, formation energy and band gap energy for the pure and doped MoS2 monolayer. Systems
Pure B-doped P-doped Cl-doped a b c
Bond length (Å) MoeXa
2.41 (2.40)b 2.07/2.08/2.18 2.41 2.53
Mulliken charge (e) X
Mo
XeMo
e 0.25 0.12 0.06
0.05 0.12/0.14 0.00 0.07
(0.37)c 0.42/0.45/0.46 0.44 0.38
Eform/eV
Eg/eV
e 3.01 1.25 0.70
1.72 1.51/1.34 1.67/1.62 1.74/1.69
X represent doped atom. Ref [45]. Charge of MoeS bond.
Table 3 The bond length of NO adsorption on pure and doped monolayer MoS2, the shortest distance of NO to the substrate surface, the adsorption energy as well as the band gap of all systems. Adsorption systems
Pure B-doped P-doped Cledoped NO a
Bond length (Å)
Distance (Å)
XeMo
NeO
XeN
2.40 2.27/2.28 2.37/2.41/2.43 2.48/2.51/2.50
1.20 1.21 1.22 1.20 1.19 1.16exp
2.89 1.31 1.91 2.72
Eads/eV
Eg/eV
0.23(0.21)a 3.49 1.29 0.27
1.72 1.20 1.03 1.50/1.69
Theoretical value with LDA method Ref [29].
3. Results and discussion 3.1. Geometrical structures of pure and doped MoS2 Bulk MoS2 is a triangular prismatic crystal cell. Analogous to graphene, it belongs to the D6h [42] crystal space group and has an indirect optical band gap of 1.29 eV. Intriguingly, owing to the quantum confinement, monolayer MoS2 nanosheets that are exfoliated from the bulk states have a direct and wider optical band gap of 1.90 eV [9]. As the number of layers decreases, monolayer MoS2 converts to the D3h [43] space group, which lacks inversion symmetry. MoS2 are closely packed layers structure, and each layer consists of covalently bonded SeMoeS planes, just like a sandwich (Fig. 1). The calculated MoeS bond length is 2.40 Å, and perpendicular length of SeS is 3.18 Å, which is consistent with the experimental result (3.16 Å) [44].
After we substituted the optimized geometry, only the B-doped MoS2 exhibited relative larger relaxation, with the B atom being pressed into the S plane (Fig. 1). Considering the structural parameters in Table 2, the lengths of the MoeX (X ¼ B, P, Cl) bonds exhibit the following general order: MoeB < MoeS (pure); MoeP;
K. Ding et al. / Vacuum 130 (2016) 146e153
the Mo atom captured electrons. Moreover, the charge population on the MoeP bond is 0.44 e, and that on the MoeB bond is 0.42e0.46 e, indicating that the covalent interactions of the MoeP and MoeB bonds are stronger than that of the pristine MoeS bond (0.37 e). And the MoeCl bond shares 0.38e, exhibit more ionicbonding characteristics. This result may explain the observed sequence of MoeCl bond lengths. Although the calculated Eform values in Table 2 reveal that the smaller value of the Cl-doped case is energetically more favorable for incorporating into MoS2 while the B and P dopants are less so. 3.2. NO adsorption on pure and doped MoS2 The most stable configurations of the NO molecule on pure and doped MoS2 are shown in Fig. 2, where a is pure MoS2, and b, c, and d are MoS2 doped with B, P and Cl, respectively. For more detailed information, the bond lengths, adsorption distance, and related adsorption energies are shown in Table 3. After full geometrical
149
relaxation, the TX site was found to be the most favorable position for NO, although pure MoS2 prefers to attach NO at the B site (Bond), which is consistent with other theoretical results [29]. The N end, rather than the O end of NO molecule is more stable to attach the MoS2 surface. Eventually, NO lies almost parallel to the MoS2 plane, with the adsorption length of NeS for 2.89 Å and that of OeS for 3.58 Å (Fig. 2a). The calculated NeO bond length of free NO molecule is 1.19 Å, while after adsorption, the length is stretched to 1.20 Å, indicating some extent of activation. As the molecular orbital of NO is KK(s2S)2 (s2s )2 (p2py)2 (p2pz)2 (s2px)2 (p2p )1, the highest occupied energy orbital of p2p has one unpaired electron, which is likely to captures electron to form a pair. Acquiring from the Mulliken charge transfer summarized in Table 4, NO acts as an electron accepter and gains electrons for 0.05 jej from MoS2, only exerting slight influence on MoS2. Moreover, the adsorption energy (0.23 eV) demonstrates that monolayer MoS2 has weak physical absorption to NO. Therefore, we hope to make some modification
Fig. 2. Top and side views of the most stable configurations and distance (Å) for NO adsorption on (a) pure, (b) B-doped, (c) P-doped, and (d) Cl-doped MoS2. The N, O atoms are deep blue and red balls respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 4 Charge transfer(e) for main atom and NO molecule in the Pure and Doped MoS2 systems before and after adsorption with NO. Systems
Pure B-doped P-doped Cl-doped a
Charge transfer after adsorption (e) X
Mo
N
O
NO
e 0.08 0.20 0.08
0.01/0.02 0.03/0.02 0.03/0.01 0
0.05 0.18 0.21 0.03
0 0.02 0 0.03
0.05a 0.20 0.21 0.06
The negative value means electrons transfer from substrate to NO molecule.
on MoS2 surface to improve the NO sensitivity. As expected, we can observe drastic changes in the B-MoS2 adsorption configuration. The NO molecule orients with its N pointing to the B of B-MoS2, similar to the orientation of NO on Bgraphene [22]. B-MoS2 strongly adsorbs NO that a new bond forms between the B and N atoms, and the tight length of BeN is 1.31 Å, shorter than that of BeN length in BN for 1.45 Å [46], exhibiting stronger ionic-bonding characteristics. The interaction could also confirmed from the charge transfer behavior that 0.20 jej is transferred from B-MoS2 to NO, obviously exceeding than that transferred from pure MoS2 for 0.05 jej. It is crucial that the Eads value increased to 3.49 eV, indicating that B-doped MoS2 is relatively sensitive to the presence of NO molecules. Similar configuration appears on P-doped MoS2. The length of newly generated NeP bond is 1.91 Å, much longer than that of the single NeP bond (1.65e1.69 Å). Meanwhile, the NO molecule is activated, as the NeO bond length is extended to 1.22 Å. In subsequent reactions, NO may be dissociated into N and O atoms and form N2 and O2. More importantly, the adsorption energy (1.26 eV) is smaller than that in the B-MoS2 system but larger than that with Cl-MoS2. In addition, the number of electrons NO molecule gains from the P-MoS2 substrate (0.21 jej) is not less than that transferred between B-MoS2 and NO. Both illuminate that P-MoS2 has proper chemisorption towards NO. Cl-MoS2 is not so active to NO molecule, the corresponding Eads value for 0.27 eV is slightly larger than that of pure MoS2. The NO molecule approaches the Cl-MoS2 plane with the NeCl length of
2.72 Å, the distance is close to than that in pure system for 2.89 Å. The alteration in NeO bond length (1.20 Å) is not as apparent as those in B- and P-doped MoS2. Little charge of 0.06 jej is transferred from Cl-MoS2 to NO molecule, indicate that Cl-MoS2 is not as attractive to NO as B-MoS2 and P-MoS2. However, Cl-MoS2 strengthens NO adsorption than the pure MoS2 system in terms of the electron transfer and adsorption value. The different adsorption phenomenon can be analyzed from the highest occupied molecular orbitals (HOMO). In Fig. 3, a represents for NO on pure MoS2, b, c, and d are B-, P-, and Cl-doped MoS2 with NO, respectively. NO molecule on pure MoS2 presented as p2p antibonding states, which exert only minor effects on neighboring S atom orbital of MoS2 surface. However, the conditions in B-MoS2 are completely different. Fig. 3b shows that a great p bond generates between B and N atoms side by side, meanwhile some orbital coupling occur on the B atom and the adjacent Mo atoms, indicate that B-MoS2 interacts strongly with NO. In P-MoS2 system (Fig. 3c), it is evident that the shapes bonding states appeared on the doped P atom and N atom in NO, the Mo atom shows d orbital feature, suggests the occurrence of certain interactions between P-MoS2 and NO. For Cl-MoS2 system, NO has little effect on Cl atom and the adjacent Mo atoms, the interaction is slightly stronger than that observed in pure MoS2. Thus, we aimed to explore whether Bdoped MoS2, which exhibited the strongest adsorption, is the best choice for MoS2-based sensors. According to the conventional transition state theory, the recovery time t of sensor devices is expressed as:
tfexpðEads =KB TÞ
(3)
where T is the temperature, and KB is the Boltzmann constant. From the expression we can infer that: more negative Eads values would result in an exponential extension of the recovery time. The strong adsorption of NO on B-MoS2 should be advantageous. However, instead, it substantially increases the recovery time needed for NO desorption, although this can be mitigated by applying an electric field to reactivate the sensing material [47]. While the weaker chemisorption of NO on P-MoS2 will make it recover more quickly.
Fig. 3. HOMO molecular orbitals of all systems with NO, a is pure MoS2, b, c, d are B-doped, P-doped and Cl-doped MoS2 respectively.
K. Ding et al. / Vacuum 130 (2016) 146e153
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Fig. 4. a1, b1, c1 d1 are the total density of states (DOS) and projected density of states (PDOS) plot for the pure and B-, P- and Cl-doped MoS2 systems; a2, b2, c2 d2 are their DOS plot with NO adsorption.
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3.3. Changes in electronic structure To better verify the effects of NO adsorption, the total and projected DOSs are plot in Fig. 4, where a1 is the DOS of pure MoS2 and free NO; b1, c1, and d1 are the DOSs of B-, P- and Cl-doped MoS2, respectively; and a2, b2, c2, and d2 are the DOSs of pure, B-, P- and Cl-doped MoS2 with adsorbed NO. In pure MoS2 (Fig. 4a1), the upper valence band is at the Fermi energy level, where the DOS are contributed by the hybridization of the Mo 4d and S 3p states, and the bottom conduction band is at 1.72 eV, where the main DOS are dominated by the Mo 4d states. After NO adsorption, the whole orbitals of MoS2 shifted to occupied states direction for about 0.98 eV, reveals the reaction is exothermic. However, the DOS of NO are almost unchangeable in contrast to that of free NO molecule (Fig. 4a1), the localized spinsplitting peak casued by N and O atom still exist at 0e1.0 eV. In addition, the unaltered band gap value of pure MoS2 (1.72 eV) after adsorption (Fig. 4a1 and a2) also indicate that the presence of NO does not affect the electronic properties of MoS2. In the substituted systems, because the valence electrons of B and P atoms are both less than that of S atom and Cl atom more valence electrons, the corresponding band structure is divided into spin-up and spin-down states and the band gap is generally narrowed with respect to the pure monolayer MoS2. For B-doped MoS2, the Eg values of the spin-up and spin-down states are 1.51 and 1.34 eV, respectively. An acceptor level appears in the spin-down states and locates above the Fermi energy for about 0.22 eV (Fig. 4b1), which is mainly attribute to the contribution of the B 2p orbital, and it is a good springboard for activated electrons in the valence band transfer to the conduction band more easily. However, after adsorption, the spin-splitting states vanish, displaying in Fig. 4 b2. Instead, a new level is introdfuced below the conduction band and reduces the band gap to 1.20 eV, which is originated from the hybrid interaction by p states of N, O and S atom. The DOS of NO undergo obvious changes, the spin polarization vanished and the peak merge to one at around 1.35 eV compare to that of isolated NO molecule (Fig. 4a1). Meanwhile, the peak of B 2p states downshift to lower energy direction compare to Fig. 4b1, and the peak of Mo 4d states almost fades away, illustrating certain interaction of B and Mo atom. In P-doped MoS2, an impurity level is induced close to the upmost valence band for the spin-down states (above the Fermi energy level for about 0.10 eV), which is mainly originated from the P 3p states. The band gap change slightly to 1.67 and 1.62 eV for the spin-up and spin-down states respectively (Fig. 4 c1). After adsorption, the spin-splitting states disappeared. And the orbital of new impurity level shifts above the Fermi energy level for about 0.60 eV (Fig. 4 c2), which is dominated by the 2p states of N and O atom, and little for Mo 4d and S 3p states. The band gap is decreased to 1.03 eV. Different from that observed in B-MoS2, the orbital hybrid interaction of NO with MoS2 in P-MoS2 is rather small, namely the interactions are weaker. Thus the NO desorption from P-MoS2 substrate would be easier. In addition, the orbital coupling of NO, P, Mo and S atoms all make contributions to the Fermi energy level. Besides, the DOS value of the P 3p states reduces significantly after adsorption, possibly because the P atom donates electrons to the adjacent atoms, especially for the N atom (Table 4). It is evident that doping Cl into MoS2 relatively shifts the total DOS toward lower energy for approximately 1.6 eV (Fig. 4 d1). The Eg values of the spin-up and spin-down states are 1.74 and 1.69 eV, respectively. However, the NO electronic levels barely contribute to the Cl-MoS2, and the spin-splitting states persist after adsorption (Fig. 4 d2). The band gap decreases to 1.50 eV in the spin-up states and remains unchanged (1.69 eV) in the spin-down states. Nonetheless, the overlapping of N and O 2p states in spin up states, Mo
4d and S 3p states and little Cl 3p states dominant at the Fermi energy level, demonstrates the strengthened adsorption. Upon adsorption, the variation of band gap is critically related to the electrical conductivity of semiconductors as follow equation [48]:
sfexpðEg =2kTÞ
(4)
where s is the electrical conductance, and k is the Boltzmann constant. According to this equation, at a given temperature, smaller Eg values lead to higher conductivity. Thus, after adsorption, the considerable reduction of the MoS2 band gap resulting from B and P doping would significantly enhance the conductivity. Based on our analysis of the electronic properties, band gap alteration and the relationship between adsorption energy and recovery time, we suggest that P doping may be more appropriate than B or Cl doping for the modification of pure MoS2 for NO sensing. 4. Conclusion In summary, by performing the density functional theory (DFT) calculations, we systematically investigate the response of NO molecule adsorption on pure and doped monolayer MoS2. Pure MoS2 exhibits weak physisorption of NO (0.23 eV) with little electron transferring, which only creating little changes in conduction. However, the chemisorption effect is more visible and thermally stable than that of physisorption. Based on our analysis of the electronic properties, charge transfer, and variations in the band gap and corresponding adsorption energy, we found that B-MoS2 interacted with NO molecules so strongly (3.49 eV) that NO desorption was difficult, even the band gap are obvious narrowed after adsorption which could significantly enhance the conductivity of the material; thus, B-doped MoS2 is presumably not suitable for gas-sensing applications. In addition, the Eads value of 0.27 eV suggests that Cl doping may not be reactive enough. Besides, rare electrons are transferred from Cl-MoS2 to NO. In contrast, P-doped MoS2 exhibited moderate NO adsorption (1.29 eV), and after adsorption the band gap are also narrowed, thus would improve the electrical conductiuity. Therefore, P-doped appears to be a more appropriate than B- or Cldoped MoS2 for NO gas detection. We hope that these results provide direct guidance for the development of novel MoS2-based gas sensors. Acknowledgements This research is supported by the National Natural Science Foundation of China (21171039). References [1] Y. Dan, Y. Lu, N.J. Kybert, Z. Luo, A.T.C. Johnson, Intrinsic response of graphene vapor sensors, Nano Lett. 9 (2009) 1472e1475. [2] A. Ghosh, D.J. Late, L.S. Panchakarla, A. Govindaraj, C.N.R. Rao, NO2 and humidity sensing characteristics of few-layer graphenes, J. Exp. Nanosci. 4 (2009) 313e322. [3] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652e655. [4] G. Korotcenkov, Metal oxides for solid-state gas sensors: what determines our choice? Mater. Sci. Eng. B 139 (2007) 1e23. [5] W. Niu, W.U. Xiaoye, S. Mei, N.A. Xingbo, L.I. Huawei, Y. Jia, Ozone detection in the ppb range with HSGFET ozone sensors operating at room temperature, J. Transcluction Technol. 1 (2001) 23e25. [6] H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D.W. Fam, A.I. Tok, Q. Zhang, H. Zhang, Fabrication of single- and multilayer MoS2 film-based field-effect transistors for sensing NO at room temperature, Small 8 (2012) 63e67. [7] F.K. Perkins, A.L. Friedman, E. Cobas, P.M. Campbell, G.G. Jernigan, B.T. Jonker,
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The enhancement of NO detection by doping strategies on monolayer MoS2 Kaining Ding*, Yihua Lin, Mengyue Huang Department of Chemistry, Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian, 350108, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 30 April 2016 Accepted 3 May 2016 Available online 4 May 2016
First principles calculations are carried out to investigate the reactivities of pure monolayer MoS2 nanosheet doped with non-metal elements X (X ¼ B, P, Cl) in the presence of NO. All doped systems exhibit strong effects on NO, especially for the B- and P-doped MoS2 cases, whereas the pure MoS2 shows weak physisorption of NO. To fully explore the mechanism of strengthening interactions between the substrate and NO, we discuss the changes in the electronic structure, which determines the electrical conductivity. We suggest that P-doped MoS2 is a suitable candidate for sensing NO polluting gas, while the chemisorption of NO on B-doped MoS2 is so strong that NO desorption is difficult. © 2016 Elsevier Ltd. All rights reserved.
Keywords: MoS2 monolayer Doped system Electronic structure Gas sensor
1. Introduction Few-layered MoS2, which is a graphene-like two-dimensional (2D) layered transition metal dichalcogenide material, has attracted enormous interest for its promising semiconducting characteristics and the advantageous band gap than graphene. Graphene, the most well-known 2D material, has been the focus of extensive attention for a long time, including as gas sensor material [1e3]. However, its zero band gap, which causes over high charge carrier mobility, limits its applications. In contrast, the unique monolayer MoS2 intriguingly shows a desirable and direct band gap of 1.90 eV. And the intrinsic semiconductor nature [4,5] and large surface-tovolume ratio make monolayer MoS2 appropriate in chemical sensors [6,7]. Owing to the distinctive physical, electronic and optical properties, MoS2 has become an important candidate in nanoelectronic and optoelectronic fields like transistors [8], photoemitting devices [9], high-performance catalysis [10,11], dry lubrication [12,13], batteries [14,15] and photovoltaics [16,17]. Toxic NO gas is known to be a highly important air pollutant. A certain concentration of NO can cause serious respiratory and skin problems among people. Therefore, materials able to detect NO are
* Corresponding author. E-mail address: [email protected] (K. Ding). http://dx.doi.org/10.1016/j.vacuum.2016.05.005 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
urgently needed. Multilayer MoS2 films based on transistor sensors have been experimentally demonstrated to show stable sensitivity towards NO gas molecules [6], whereas single-layer MoS2 films exhibit sharp but unstable current responses. For this reason, we are interested in decorating the surface of monolayer MoS2 to improve its chemical sensitivity to NO. Impurity doping may be a desirable approach to modulate the electronic properties of monolayer MoS2. Qin et al. synthesized Ndoped MoS2 nanosheets by the sol-gel approach [18]; Yang et al. experimentally doped 2D WS2 and MoS2 with chlorine molecules and concluded that Cl doping could be applied in the fabrication of high-performance electronic devices [19]. Liu et al. prepared B- and N-doped MoS2 nanosheets as fluorescent probes to detect the heavy metal ion Hg2þ with high sensitivity and selectivity [20]. Yue et al. theoretically substituted non-metal atoms (H, B, C, O, N, and F) and transition metal atoms into MoS2 sheets in VS defects [21] and found that all substituted atoms are favorable bound to VS defects. Various discoveries regarding the surface modification of nanostructured materials have been reported. For example, B- and S-doped graphenes have been shown to chemically bind NO2 [22]. Zhang et al. demonstrated that using B-doped graphene could significantly improve the sensitivity and selectivity of gas sensors [23]. Incorporating Au into SnO2 thick films was found to be a promising method to improve the CO gas response [24] Al-doped SnO2 thin films obtained via the rheotaxial growth and thermal oxidation (RGTO) method also exhibited enhanced gas sensing [25].
K. Ding et al. / Vacuum 130 (2016) 146e153
In addition, Si-doped SnO2 films are useful liquid petroleum gas sensors [26]. Defects can significantly change the electronic properties of h-AlN sheets [27]. F-doped corannulene molecules are more stable than H-doped (hydrogenated) ones for potential applications in C-based nanostructures [28]. Few studies have investigated the adsorption of small gas molecules on monolayer MoS2. Yue et al. showed that different molecules, such as NH3, NO, and NO2, only physisorbed to monolayer MoS2 with a small charge transfer, which could be significantly modulated by a perpendicular electric field [29]. Zhao et al. also reported that among small gas molecules, only NO and NO2 exhibited stronger binding to MoS2 sheets [30]. Both of the studies provide a theoretical basis for the potential application of MoS2 monolayers as polluting gas sensors for gases like NO and NO2. However, a systematic investigation into responses obtained by exposing doped and perfect MoS2 nanosheets to NO gas has not been undertaken. Here, to choose the best dopants, we explored NO gas adsorption by doping a series of non-metals (B, C, N, O, F, Si, P and Cl) into MoS2. We found that C-, Si-, N-doped MoS2 had similar adsorption activities to P-doped MoS2, that O and F doping caused almost no change in the NO adsorption, and that the effect of Cl doping was slightly better than those of O and F. The B-doped case showed the strongest adsorption. Hence, we selected B, P, and Cl as dopants according to the observed adsorption strengths. In addition, we expect that B may be the potential candidate like its application in B-graphene based gas sensors. P and Cl atoms are at the same period as S in the periodic table. The covalent radius of B, P, and Cl are approximately equivalent to that of S, and thus, B, P and Cl readily form stable bonds with Mo. Most importantly, their p orbitals contain odd numbers of electrons, and therefore, changes in their electronic densities of states (DOSs; spin-splitting states) can be clearly compared. We used the doped MoS2 to detect NO gas and explored the possibilities of transforming the gas adsorption from physisorption to certain types of chemical adsorption. Although physisorption can create changes in conduction, the effects of chemisorption are relatively visible and thermally stable [22]. By calculating the adsorption energies and exploring the changes in the electronic properties and charge transfer, we discuss the interaction mechanisms between the substrates and NO and their effects on the sensing properties. We expect that these results will constitute a solid foundation for designing gas-sensing devices. 2. Computational method and details Our calculations are carried out by density functional (DFT) theory based on the first-principles and performed by the Cambridge Serial Total Energy Package (CASTEP) code [31]. The generalized gradient approximation of Perdew-Wang 91 gradientcorrected function (GGA-PW91) [32] is used to optimize the geometrical configurations of pure and doped MoS2. Considering the large size effect,we construct the 4 4 supercell for monolayer MoS2 which consists of 32 sulfur atoms and 16 molybdenum atoms. To avoid interlayer interactions, the vacuum thickness was set to 18 Å. After testing, we set the Brillouin zone sampling k-points meshes as 8 8 1 and kinetic cut-off energy for the plane-wave expansion as 440 eV. The valence atomic electrons configurations are 3s23p4 for S, 4d55s1 for Mo, and 2s22p1 for B, 2s22p3 for N, 2s22p4 for O, 3s23p3 for P and 3s23p5 for Cl. The convergence criterion of the energy change, the maximum forces, the stresses on the cell, the atomic displacements were set as 2 105 eV, 0.05 eV/ Å, 0.1 GPa and 2 103 Å. To acquire the appropriate method to explore the electronic structure for these systems, GGA-PW91 [32], GGA-PBE [33], the local density approximation (LDA) [34], the hybrid HSE06 [35]
147
function are all employed to calculate the band gap of pure MoS2. Based on the results from different functions, we found that hybrid HSE06 function always gives relatively exact results but excessively overestimate the value of band gap in our MoS2 systems. The remained LDA, GGA-PW91, GGA-PBE method give the similar value for MoS2 band gap. The LDA often overestimated the adsorption energy [29], and the band gap value calculated by GGA-PW91 for 1.72 eV is more close to experimental results (1.90 eV) [9] than that calculated by GGA-PBE for 1.71 eV, the results could reference to Table 1. Therefore, we finally choose the GGA-PW91 method. To prevent the cases of GGA density approximation cannot entirely achieve the weakly bonded van der Waals (vdW) interactions for gas molecule adsorption on monolayer MoS2, we additionally select the OBS method for DFT-D correction. There are also some theoretical studies adopting the OBS method for DFT-D correction to consider the effect of the van der Waals interactions, Ref [36,37]. Compared to conventional DFT calculations, DFT-D method increases the semiempirical pair-wise force field and ensures to correctly account for the effect of vdW interactions. For doping in S vacancy is easier to bind than in Mo vacancy [21], we use single B, P or Cl atom to replace one S atom of MoS2 in TX site, see Fig. 1. The corresponding doping concentration is 2.08%. And the doping formation energies are calculated by the following formula:
Eform ¼ Edoped þ mS EMoS2 mX
(1)
Edoped are total energies of MoS2 doped with B, P, or Cl, and EMoS2 is the total energies for pure MoS2. mx are chemical potentials of B, P and Cl. And mX are taken from their most stable phases, mS ¼ [E(H2S)eE(H2)], mB ¼ 1/2 [E(B2H6)3E(H2)], mP ¼ 1/4 E(P4), mCl ¼ 1/2 E(Cl2). The calculated results are summarized in Table 2. To obtain the most stable optimal adsorption configuration, we considered five adsorption sites. First the TX site on top of the doped site; second, the TM site on top of a Mo atom; third, the TS site on top of a S atom; and fourth, the B site on top of a MoeS bond and the last H site on top of the hollow for a hexagon while in top view. We placed three different gas orientations at each site, one is the NO axis lies parallel to the monolayer. The other two are the NO molecule perpendicular to these sites, with N or O atom pointing downward. And the adsorption energy of gas molecules on MoS2 is defined as:
Eads ¼ ENO=system Esystem þ ENO
(2)
Esystem is the total energy for pure or doped MoS2. And ENO/system represents for the total energy of the system adsorbed with NO. ENO is the total energy of isolated NO molecule. The negative value of Eads indicates the adsorption process is exothermic, likewise, the calculated results are summarized in Table 3.
Table 1 Calculation of single energy and band gap with different methods. Method
GGA/PW91
GGA/PBE
LDA
HSE06
Exp
Eg/eV
1.72 1.70a
1.71 1.67b
1.73 1.87c
2.25 2.21d
1.90e
a b c d e
Ref Ref Ref Ref Ref
[38]. [39]. [40]. [41]. [9].
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Fig. 1. a is the top view of MoS2 monolayer,the marked red rose balls represent for the adsorption site of TX, TM, H and B0 (bond); b, c, d are the side view of B, P, Cl doped MoS2 monolayer in TX site. Each type of atoms has been color coded. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Structural relaxtion parameters, Mulliken charge population (e) on the main atom and bonds, formation energy and band gap energy for the pure and doped MoS2 monolayer. Systems
Pure B-doped P-doped Cl-doped a b c
Bond length (Å) MoeXa
2.41 (2.40)b 2.07/2.08/2.18 2.41 2.53
Mulliken charge (e) X
Mo
XeMo
e 0.25 0.12 0.06
0.05 0.12/0.14 0.00 0.07
(0.37)c 0.42/0.45/0.46 0.44 0.38
Eform/eV
Eg/eV
e 3.01 1.25 0.70
1.72 1.51/1.34 1.67/1.62 1.74/1.69
X represent doped atom. Ref [45]. Charge of MoeS bond.
Table 3 The bond length of NO adsorption on pure and doped monolayer MoS2, the shortest distance of NO to the substrate surface, the adsorption energy as well as the band gap of all systems. Adsorption systems
Pure B-doped P-doped Cledoped NO a
Bond length (Å)
Distance (Å)
XeMo
NeO
XeN
2.40 2.27/2.28 2.37/2.41/2.43 2.48/2.51/2.50
1.20 1.21 1.22 1.20 1.19 1.16exp
2.89 1.31 1.91 2.72
Eads/eV
Eg/eV
0.23(0.21)a 3.49 1.29 0.27
1.72 1.20 1.03 1.50/1.69
Theoretical value with LDA method Ref [29].
3. Results and discussion 3.1. Geometrical structures of pure and doped MoS2 Bulk MoS2 is a triangular prismatic crystal cell. Analogous to graphene, it belongs to the D6h [42] crystal space group and has an indirect optical band gap of 1.29 eV. Intriguingly, owing to the quantum confinement, monolayer MoS2 nanosheets that are exfoliated from the bulk states have a direct and wider optical band gap of 1.90 eV [9]. As the number of layers decreases, monolayer MoS2 converts to the D3h [43] space group, which lacks inversion symmetry. MoS2 are closely packed layers structure, and each layer consists of covalently bonded SeMoeS planes, just like a sandwich (Fig. 1). The calculated MoeS bond length is 2.40 Å, and perpendicular length of SeS is 3.18 Å, which is consistent with the experimental result (3.16 Å) [44].
After we substituted the optimized geometry, only the B-doped MoS2 exhibited relative larger relaxation, with the B atom being pressed into the S plane (Fig. 1). Considering the structural parameters in Table 2, the lengths of the MoeX (X ¼ B, P, Cl) bonds exhibit the following general order: MoeB < MoeS (pure); MoeP;
K. Ding et al. / Vacuum 130 (2016) 146e153
the Mo atom captured electrons. Moreover, the charge population on the MoeP bond is 0.44 e, and that on the MoeB bond is 0.42e0.46 e, indicating that the covalent interactions of the MoeP and MoeB bonds are stronger than that of the pristine MoeS bond (0.37 e). And the MoeCl bond shares 0.38e, exhibit more ionicbonding characteristics. This result may explain the observed sequence of MoeCl bond lengths. Although the calculated Eform values in Table 2 reveal that the smaller value of the Cl-doped case is energetically more favorable for incorporating into MoS2 while the B and P dopants are less so. 3.2. NO adsorption on pure and doped MoS2 The most stable configurations of the NO molecule on pure and doped MoS2 are shown in Fig. 2, where a is pure MoS2, and b, c, and d are MoS2 doped with B, P and Cl, respectively. For more detailed information, the bond lengths, adsorption distance, and related adsorption energies are shown in Table 3. After full geometrical
149
relaxation, the TX site was found to be the most favorable position for NO, although pure MoS2 prefers to attach NO at the B site (Bond), which is consistent with other theoretical results [29]. The N end, rather than the O end of NO molecule is more stable to attach the MoS2 surface. Eventually, NO lies almost parallel to the MoS2 plane, with the adsorption length of NeS for 2.89 Å and that of OeS for 3.58 Å (Fig. 2a). The calculated NeO bond length of free NO molecule is 1.19 Å, while after adsorption, the length is stretched to 1.20 Å, indicating some extent of activation. As the molecular orbital of NO is KK(s2S)2 (s2s )2 (p2py)2 (p2pz)2 (s2px)2 (p2p )1, the highest occupied energy orbital of p2p has one unpaired electron, which is likely to captures electron to form a pair. Acquiring from the Mulliken charge transfer summarized in Table 4, NO acts as an electron accepter and gains electrons for 0.05 jej from MoS2, only exerting slight influence on MoS2. Moreover, the adsorption energy (0.23 eV) demonstrates that monolayer MoS2 has weak physical absorption to NO. Therefore, we hope to make some modification
Fig. 2. Top and side views of the most stable configurations and distance (Å) for NO adsorption on (a) pure, (b) B-doped, (c) P-doped, and (d) Cl-doped MoS2. The N, O atoms are deep blue and red balls respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 4 Charge transfer(e) for main atom and NO molecule in the Pure and Doped MoS2 systems before and after adsorption with NO. Systems
Pure B-doped P-doped Cl-doped a
Charge transfer after adsorption (e) X
Mo
N
O
NO
e 0.08 0.20 0.08
0.01/0.02 0.03/0.02 0.03/0.01 0
0.05 0.18 0.21 0.03
0 0.02 0 0.03
0.05a 0.20 0.21 0.06
The negative value means electrons transfer from substrate to NO molecule.
on MoS2 surface to improve the NO sensitivity. As expected, we can observe drastic changes in the B-MoS2 adsorption configuration. The NO molecule orients with its N pointing to the B of B-MoS2, similar to the orientation of NO on Bgraphene [22]. B-MoS2 strongly adsorbs NO that a new bond forms between the B and N atoms, and the tight length of BeN is 1.31 Å, shorter than that of BeN length in BN for 1.45 Å [46], exhibiting stronger ionic-bonding characteristics. The interaction could also confirmed from the charge transfer behavior that 0.20 jej is transferred from B-MoS2 to NO, obviously exceeding than that transferred from pure MoS2 for 0.05 jej. It is crucial that the Eads value increased to 3.49 eV, indicating that B-doped MoS2 is relatively sensitive to the presence of NO molecules. Similar configuration appears on P-doped MoS2. The length of newly generated NeP bond is 1.91 Å, much longer than that of the single NeP bond (1.65e1.69 Å). Meanwhile, the NO molecule is activated, as the NeO bond length is extended to 1.22 Å. In subsequent reactions, NO may be dissociated into N and O atoms and form N2 and O2. More importantly, the adsorption energy (1.26 eV) is smaller than that in the B-MoS2 system but larger than that with Cl-MoS2. In addition, the number of electrons NO molecule gains from the P-MoS2 substrate (0.21 jej) is not less than that transferred between B-MoS2 and NO. Both illuminate that P-MoS2 has proper chemisorption towards NO. Cl-MoS2 is not so active to NO molecule, the corresponding Eads value for 0.27 eV is slightly larger than that of pure MoS2. The NO molecule approaches the Cl-MoS2 plane with the NeCl length of
2.72 Å, the distance is close to than that in pure system for 2.89 Å. The alteration in NeO bond length (1.20 Å) is not as apparent as those in B- and P-doped MoS2. Little charge of 0.06 jej is transferred from Cl-MoS2 to NO molecule, indicate that Cl-MoS2 is not as attractive to NO as B-MoS2 and P-MoS2. However, Cl-MoS2 strengthens NO adsorption than the pure MoS2 system in terms of the electron transfer and adsorption value. The different adsorption phenomenon can be analyzed from the highest occupied molecular orbitals (HOMO). In Fig. 3, a represents for NO on pure MoS2, b, c, and d are B-, P-, and Cl-doped MoS2 with NO, respectively. NO molecule on pure MoS2 presented as p2p antibonding states, which exert only minor effects on neighboring S atom orbital of MoS2 surface. However, the conditions in B-MoS2 are completely different. Fig. 3b shows that a great p bond generates between B and N atoms side by side, meanwhile some orbital coupling occur on the B atom and the adjacent Mo atoms, indicate that B-MoS2 interacts strongly with NO. In P-MoS2 system (Fig. 3c), it is evident that the shapes bonding states appeared on the doped P atom and N atom in NO, the Mo atom shows d orbital feature, suggests the occurrence of certain interactions between P-MoS2 and NO. For Cl-MoS2 system, NO has little effect on Cl atom and the adjacent Mo atoms, the interaction is slightly stronger than that observed in pure MoS2. Thus, we aimed to explore whether Bdoped MoS2, which exhibited the strongest adsorption, is the best choice for MoS2-based sensors. According to the conventional transition state theory, the recovery time t of sensor devices is expressed as:
tfexpðEads =KB TÞ
(3)
where T is the temperature, and KB is the Boltzmann constant. From the expression we can infer that: more negative Eads values would result in an exponential extension of the recovery time. The strong adsorption of NO on B-MoS2 should be advantageous. However, instead, it substantially increases the recovery time needed for NO desorption, although this can be mitigated by applying an electric field to reactivate the sensing material [47]. While the weaker chemisorption of NO on P-MoS2 will make it recover more quickly.
Fig. 3. HOMO molecular orbitals of all systems with NO, a is pure MoS2, b, c, d are B-doped, P-doped and Cl-doped MoS2 respectively.
K. Ding et al. / Vacuum 130 (2016) 146e153
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Fig. 4. a1, b1, c1 d1 are the total density of states (DOS) and projected density of states (PDOS) plot for the pure and B-, P- and Cl-doped MoS2 systems; a2, b2, c2 d2 are their DOS plot with NO adsorption.
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K. Ding et al. / Vacuum 130 (2016) 146e153
3.3. Changes in electronic structure To better verify the effects of NO adsorption, the total and projected DOSs are plot in Fig. 4, where a1 is the DOS of pure MoS2 and free NO; b1, c1, and d1 are the DOSs of B-, P- and Cl-doped MoS2, respectively; and a2, b2, c2, and d2 are the DOSs of pure, B-, P- and Cl-doped MoS2 with adsorbed NO. In pure MoS2 (Fig. 4a1), the upper valence band is at the Fermi energy level, where the DOS are contributed by the hybridization of the Mo 4d and S 3p states, and the bottom conduction band is at 1.72 eV, where the main DOS are dominated by the Mo 4d states. After NO adsorption, the whole orbitals of MoS2 shifted to occupied states direction for about 0.98 eV, reveals the reaction is exothermic. However, the DOS of NO are almost unchangeable in contrast to that of free NO molecule (Fig. 4a1), the localized spinsplitting peak casued by N and O atom still exist at 0e1.0 eV. In addition, the unaltered band gap value of pure MoS2 (1.72 eV) after adsorption (Fig. 4a1 and a2) also indicate that the presence of NO does not affect the electronic properties of MoS2. In the substituted systems, because the valence electrons of B and P atoms are both less than that of S atom and Cl atom more valence electrons, the corresponding band structure is divided into spin-up and spin-down states and the band gap is generally narrowed with respect to the pure monolayer MoS2. For B-doped MoS2, the Eg values of the spin-up and spin-down states are 1.51 and 1.34 eV, respectively. An acceptor level appears in the spin-down states and locates above the Fermi energy for about 0.22 eV (Fig. 4b1), which is mainly attribute to the contribution of the B 2p orbital, and it is a good springboard for activated electrons in the valence band transfer to the conduction band more easily. However, after adsorption, the spin-splitting states vanish, displaying in Fig. 4 b2. Instead, a new level is introdfuced below the conduction band and reduces the band gap to 1.20 eV, which is originated from the hybrid interaction by p states of N, O and S atom. The DOS of NO undergo obvious changes, the spin polarization vanished and the peak merge to one at around 1.35 eV compare to that of isolated NO molecule (Fig. 4a1). Meanwhile, the peak of B 2p states downshift to lower energy direction compare to Fig. 4b1, and the peak of Mo 4d states almost fades away, illustrating certain interaction of B and Mo atom. In P-doped MoS2, an impurity level is induced close to the upmost valence band for the spin-down states (above the Fermi energy level for about 0.10 eV), which is mainly originated from the P 3p states. The band gap change slightly to 1.67 and 1.62 eV for the spin-up and spin-down states respectively (Fig. 4 c1). After adsorption, the spin-splitting states disappeared. And the orbital of new impurity level shifts above the Fermi energy level for about 0.60 eV (Fig. 4 c2), which is dominated by the 2p states of N and O atom, and little for Mo 4d and S 3p states. The band gap is decreased to 1.03 eV. Different from that observed in B-MoS2, the orbital hybrid interaction of NO with MoS2 in P-MoS2 is rather small, namely the interactions are weaker. Thus the NO desorption from P-MoS2 substrate would be easier. In addition, the orbital coupling of NO, P, Mo and S atoms all make contributions to the Fermi energy level. Besides, the DOS value of the P 3p states reduces significantly after adsorption, possibly because the P atom donates electrons to the adjacent atoms, especially for the N atom (Table 4). It is evident that doping Cl into MoS2 relatively shifts the total DOS toward lower energy for approximately 1.6 eV (Fig. 4 d1). The Eg values of the spin-up and spin-down states are 1.74 and 1.69 eV, respectively. However, the NO electronic levels barely contribute to the Cl-MoS2, and the spin-splitting states persist after adsorption (Fig. 4 d2). The band gap decreases to 1.50 eV in the spin-up states and remains unchanged (1.69 eV) in the spin-down states. Nonetheless, the overlapping of N and O 2p states in spin up states, Mo
4d and S 3p states and little Cl 3p states dominant at the Fermi energy level, demonstrates the strengthened adsorption. Upon adsorption, the variation of band gap is critically related to the electrical conductivity of semiconductors as follow equation [48]:
sfexpðEg =2kTÞ
(4)
where s is the electrical conductance, and k is the Boltzmann constant. According to this equation, at a given temperature, smaller Eg values lead to higher conductivity. Thus, after adsorption, the considerable reduction of the MoS2 band gap resulting from B and P doping would significantly enhance the conductivity. Based on our analysis of the electronic properties, band gap alteration and the relationship between adsorption energy and recovery time, we suggest that P doping may be more appropriate than B or Cl doping for the modification of pure MoS2 for NO sensing. 4. Conclusion In summary, by performing the density functional theory (DFT) calculations, we systematically investigate the response of NO molecule adsorption on pure and doped monolayer MoS2. Pure MoS2 exhibits weak physisorption of NO (0.23 eV) with little electron transferring, which only creating little changes in conduction. However, the chemisorption effect is more visible and thermally stable than that of physisorption. Based on our analysis of the electronic properties, charge transfer, and variations in the band gap and corresponding adsorption energy, we found that B-MoS2 interacted with NO molecules so strongly (3.49 eV) that NO desorption was difficult, even the band gap are obvious narrowed after adsorption which could significantly enhance the conductivity of the material; thus, B-doped MoS2 is presumably not suitable for gas-sensing applications. In addition, the Eads value of 0.27 eV suggests that Cl doping may not be reactive enough. Besides, rare electrons are transferred from Cl-MoS2 to NO. In contrast, P-doped MoS2 exhibited moderate NO adsorption (1.29 eV), and after adsorption the band gap are also narrowed, thus would improve the electrical conductiuity. Therefore, P-doped appears to be a more appropriate than B- or Cldoped MoS2 for NO gas detection. We hope that these results provide direct guidance for the development of novel MoS2-based gas sensors. Acknowledgements This research is supported by the National Natural Science Foundation of China (21171039). References [1] Y. Dan, Y. Lu, N.J. Kybert, Z. Luo, A.T.C. Johnson, Intrinsic response of graphene vapor sensors, Nano Lett. 9 (2009) 1472e1475. [2] A. Ghosh, D.J. Late, L.S. Panchakarla, A. Govindaraj, C.N.R. Rao, NO2 and humidity sensing characteristics of few-layer graphenes, J. Exp. Nanosci. 4 (2009) 313e322. [3] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652e655. [4] G. Korotcenkov, Metal oxides for solid-state gas sensors: what determines our choice? Mater. Sci. Eng. B 139 (2007) 1e23. [5] W. Niu, W.U. Xiaoye, S. Mei, N.A. Xingbo, L.I. Huawei, Y. Jia, Ozone detection in the ppb range with HSGFET ozone sensors operating at room temperature, J. Transcluction Technol. 1 (2001) 23e25. [6] H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D.W. Fam, A.I. Tok, Q. Zhang, H. Zhang, Fabrication of single- and multilayer MoS2 film-based field-effect transistors for sensing NO at room temperature, Small 8 (2012) 63e67. [7] F.K. Perkins, A.L. Friedman, E. Cobas, P.M. Campbell, G.G. Jernigan, B.T. Jonker,
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