Modulation of Gas Adsorption and Magnetic Properties of Monolayer

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Modulation of Gas Adsorption and Magnetic Properties of Monolayer-MoS by Antisite Defect and Strain 2

Mohapatra Prakash Kumar Sahoo, Jie Wang, Yajun Zhang, Takahiro Shimada, and Takayuki Kitamura J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03284 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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The Journal of Physical Chemistry

Modulation of Gas Adsorption and Magnetic Properties of Monolayer-Mos2 by Antisite Defect and Strain MPK Sahooa, Jie Wanga*, Yajun Zhanga, Takahiro Shimadab, and Takayuki Kitamurab a

Department of Engineering Mechanics, School of Aeronautics and Astronautics, Zhejiang University, Hangzhou 310027, China b

Department of Mechanical Engineering and Science, Kyoto University, Nishikyo-ku, Kyoto 615-8540, Japan

Abstract Flexible nature and high surface-to-volume ratio makes monolayer-MoS2 a novel paradigm for tunable nanoelectronic devices. However, for further improvement in the performance of these devices, a new design strategy is essential to modulate the properties of inert MoS2 basal plane. Here, we demonstrate from first principles that the gas adsorption and magnetic properties of MoS2 can be modulated through MoS antisite doping and strain. The MoS defect with localized dorbital electron density significantly promotes the catalytic activity which leads to highly enhanced adsorption of NO, NO2, NH3, CO and CO2 gas molecules. On application of a biaxial tensile strain, the adsorption of NH3 is further enhanced for the antisite doped MoS2. In addition, strain induced switching of magnetic states is also realized in antisite doped MoS2 with and without adsorbed gas species. The superior strain modulation of antisite doped MoS2 is explained by quantum confinement effect and strain induced accumulation/depletion of charge density at the defect site. These results suggest that antisite doped MoS2 can be a promising avenue to design nanoscale spintronic devices and gas sensors.

*E‐mail:[email protected] Telephone No: +86‐(0)571‐87953110

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Introduction The monolayer-MoS2 (m-MoS2) has fascinated the scientific community due to their

ultrathin structure and extraordinary electronic, optical and catalytic properties.1-4 As one of the two-dimensional (2D) materials, m-MoS2 exhibits promising electronic properties such as high carrier mobilities (200 cm2V-1s-1) at room temperature, high current on/off ratio (1×108) and ultralow standby power dissipation.5 Based on atomic layer MoS2, thin film transistors (TFT), optoelectronic devices and digital circuits have been successfully fabricated.5-8 Besides, high surface-to-volume ratio, selective reactivity upon exposure to a range of analytes, rapid response and recovery make MoS2 as a superior gas sensor with higher sensitivity and selectivity in comparison to graphene and other 2D materials.9-11 It has been reported that the mechanically cleaved single and multilayer MoS2 based field effect transistors can be used as NO gas sensor with a detection limit of 0.8 ppm.12 Flexible MoS2 TFT array on polyethylene terephthalate substrate shows high sensitivity towards NO2 sensing.

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In addition, functionalization of MoS2

with Pt nanoparticle was found to promote gas sensing activity three times higher with the detection limit of 2 ppb.13 The MoS2 film synthesized by sulfurization of sputtered Mo-thin film was found to be useful for detection of NH3 in sub-ppm level.14 Nevertheless, for further improvement in sensing capabilities and other properties, a new design strategy is essential to modulate the properties of inert MoS2 basal plane. In recent years, defect engineering has emerged as an effective strategy to tune various properties of 2D materials such as graphene, hexagonal boron nitride and silicene.15-17 Like other 2D materials, intrinsic structural defects including point defects, grain boundaries, dislocations and edge reconstructions (representing a transition state during growth) can be realized in MoS2 during the growth process.18-21 It has also been proposed that the one-dimensional MoS2 metallic wires can be created via two different types of 60o grain boundaries consisting of distinct 4-fold ring chains.21 Further, these defects can also be intentionally introduced through electron irradiation process,20 which alternatively provides an opportunity to modulate materials properties and explore new functionalities. For instance, the defects like VMoS6 vacancies can magnetically activate the MoS2 basal plane. 22 In addition, these defect sites are found to be highly reactive and could significantly promote the electronic and catalytic activity of the basal planes. The S-vacancy is found to be the most preferred catalytic site for the adsorption of nonpolar gases like CO2 and CH4, in comparison to other stoichiometric sites. 23 On the other hand, monolayer MoS2 exhibits 2


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similar mechanical properties as graphene. This strain can serve as another means to induce, modulate and optimize properties. For example, the application of biaxial strain to sulfur deficient MoS2 can inject spin moment.22 The biaxial strain can effectively tailor the band gap of pristine MoS2. 24 Also a combination of S-vacancy and strain can bring highest hydrogen evolution reaction activity in MoS2.25 Furthermore, a nonmagnetic to long range ferromagnetic transition is realized in monolayer MoS2 through strained MoS2 antisite defect.26 Though many efforts have been made to improve the gas sensing activity in MoS2 through defect engineering,12-14, 27 however very few defects have brought substantial improvement. The key factor that controls the gas sensing property is the localization of electron density at the defect site, which significantly promotes the sensing activity through mixing of orbitals followed by charge transfer.28 Recently, a new type of defect, i.e., MoS antisite defect (one of the S-atom is replaced by one Mo atom) has been experimentally explored through physical vapor deposition technique.19 This defect can be realized by creating a sulfur vacancy and doping one Mo atom at the vacancy site, which can induce a high magnetic moment (2µB) at zero strain. In addition, it is highly anticipated that the MoS defect with highly localized d-electron may bring significant enhancement to the sensitivity and selectivity towards the gas adsorption characteristics. Due to its deformation-dependent properties, application of biaxial strain may further tune the gas sensitivity and the defect induced spin moment in antisite doped MoS2. Thus, this new type of defect can be a novel avenue to realize novel or enhanced properties by combining a mechanical strain. In the present work, using first principles calculations based on density functional theory, we investigate the gas adsorption and magnetic properties of MoS2 through the strained MoS defect. It is demonstrated that the defect sites with highly accumulated charge density significantly promote the catalytic activity and exceptionally lower the adsorption energy for gas molecules such as NO, NO2, NH3, CO, CO2, in comparison to its undoped counterpart. In particular, the adsorption of NH3 in the antisite doped MoS2 is further enhanced with the presence of a biaxial tensile strain. In addition, the magnetic states in the defect doped MoS2 can be switched by the biaxial tensile strain. The Bader charge and electronic structure analyses have been successfully implemented to explore the underlying physical mechanism.

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Method We investigate the electronic, structural and energetics of monolayer MoS2 doped with MoS antisite defect (henceforth, denoted as A-MoS2) using Vienna Ab Initio Simulation Package (VASP) within the framework of density functional theory (DFT).29 To realize

antisite

doping, one of the S-atom from the relaxed 4×4×1 structure of pristine MoS2 is substituted by one Mo atom. Fig. 1 (a-d) elucidates the relaxed structures of pristine MoS2 and A-MoS2. The exchange correlation was treated by generalized gradient approximation with Perdew-Burke-Ernzerhof parameterization.30 To explore magnetic exchange interaction, both spin polarized and non-spinpolarized calculations are carried out. The Brillouin zone is sampled by 7×7×1 Monkhorst-Pack k-point mesh and the energy cutoff for plane waves is set to 500 eV. In order to avoid the interaction between MoS2 layers a vacuum layer of 15 Å is introduced in the direction normal to the MoS2 layers. All the structural optimizations are carried out until residual forces are less than 0.02 eV/Å. A biaxial strain is preferred over a uniaxial strain because of practical consideration that the strain arising from the coupling of A-MoS2 with the substrate is isotropic in nature.31 For imposing an in-plane equibiaxial strain the lattice parameter c is varied by following the relation, η = (c-c0)/c0, where the unstrained lattice constant c0 (~3.18 Å) is in good agreement with the experimental lattice constant i.e., 3.16 Å.32 For each strain, all the atoms are fully relaxed with a fixed superlattice. Results and Discussion We first discuss the electronic properties of monolayer MoS2 doped with MoS antisite defect. In undoped and unstrained MoS2, the hybridization is dominated by Mo-d and S-p orbitals.33 The incorporation of MoS antisite defect breaks the structural symmetry and drastically changes the hybridization at the defect site, yielding three mid-gap defect states (Fig. 2). Out of the three, two highly spin polarized defect states located at -0.02 and -0.11 eV below the Fermi level are found to be partially filled, whereas the third one is observed at 0.28 eV above the Fermi level is unfilled. The origin of these defect states can be explained by following crystal field theory. Under the crystal field, the degenerate energy levels of antisite Mo 4d state can splits into a single state and two twofold degenerate

(

,

and

,

states, because of

symmetry. From the orbital-resolved density of states it is observed that the unoccupied defect state above the Fermi level represents

state, whereas both the defect states below the Fermi 4


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level belongs to the

(

,

state. The two levels

state under John-Teller distortion. The

,

and

explains the split off of

state is found be occupied and present deep

into the valence band. So far as spin moment is concerned, the occupied do not contribute to the spin moment, however, the half filled 2

and unoccupied -state

-state induce a spin moment of

(discussed later). In addition to the majority spin states, minority

and

spin states with

degenerate energies are also located in the empty conduction band. Since these minority spin states are empty, thus they do not contribute to the observed spin moment. Enhanced gas adsorption of monolayer-MoS2 by antisite doping The adsorption of NH3, NO2, NO, CO and CO2 gas molecules on the surface of A-MoS2 are investigated in the present work. Since these gas molecules are exothermically adsorbed on the pristine MoS2 surface,23, 34, 35 thus studying the adsorption of these gases can provide a clear comparison between gas adsorption properties of MoS2 and A-MoS2. We begin our calculation by keeping the center of mass of gas molecules on the top of the antisite defect and then relaxing the structure. We considered several typical orientations of the gas molecules with respect to the monolayer surface. Fig. 3 (a)-(b) illustrates the most stable adsorption configuration of gas molecules on A-MoS2 surface. The adsorption energy of gas molecules on MoS2 with and without antisite defect is defined by, and

, where

,

, are the total energy of gas adsorbed MoS2 (A-MoS2), energy of MoS2 (A-MoS2) and gas

molecule, respectively. The key parameters such as adsorption distance, charge transfer and magnetic moment for pristine MoS2 and A-MoS2 are summarized in Table 1. For pristine MoS2, the obtained values of adsorption energy, adsorption distance, charge transfer and magnetic moment are in good agreement with the previous reports.23,

34, 35

The adsorption energy for

different gas species on pristine MoS2 and A-MoS2 are presented in Fig 4. It is worthwhile to note that the adsorption of gas molecule on A-MoS2 is highly exothermic (negative adsorption energy) in comparison to MoS2, suggesting that the A-MoS2 can be a better gas sensor than pristine MoS2. In addition, high negative adsorption energy for A-MoS2 also indicates a much stronger interaction between the gas molecule and the antisite defect. This distinct behavior of A-MoS2 can be ascribed to the adsorption configuration and charge transfer, systematically discussed below.

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The NH3 gas molecule is preferentially adsorbed at the defect site with its center of mass and N-atom located on the top of antisite atom. The three H-atoms are found to be displaced away from the A-MoS2 surface (Fig 3a-3b). The adsorption of NH3 gas molecule on A-MoS2 yields an adsorption energy -1.35 eV which is significantly lower than that in pristine MoS2 (i.e. -0.017 eV). The reduction in adsorption energy is attributable to the reduction of adsorption distance from 2.94 Å (dgas-MoS2) to 2.29 Å (dgas-AMoS2) (Table 1). A net charge transfer of 0.042 e is observed from the antisite atom to the gas molecule, which is in sharp contrast to its adsorption in pristine MoS2 where NH3 acts as an electron donor (discussed later). The charge distribution between the gas molecule and A-MoS2 (Fig 3c-3d) suggests that the bonding between the NH3 molecular and antisite atom is partially covalent in character.23 Besides, the adsorption of NH3 on A-MoS2 doesn’t affect the spin moment of A-MoS2 (2

). It is noteworthy that other possible adsorption positions

(such as: top of hexagon, top of S- atom, top of Mo-atom, and top of bridge site) for NH3 are highly unstable and upon structural relaxation leads to the shifting of the gas molecule to the top of the antisite defect (as described above). This also suggests that the antisite defect is a highly active catalytic site for the capture of ammonia gas. It is worth noting that for effective capture of gas molecule on solid surface the adsorption energy should be greater than -0.50 eV.36,37 The pristine MoS2 with smaller adsorption energy for NH3 (-0.017 eV) is unsuitable for fabrication of NH3 sensor. In sharp contrast, the antisite decorated MoS2 with adsorption energy -1.35 eV is highly desirable for the aforementioned purpose. The NO2 gas prefers a slightly tilted configuration with one of the O atoms comes closer to the monolayer surface. It has been observed that the adsorption of paramagnetic molecule like NO2 on nonmagnetic graphene and MoS2 induces spin moment of 1

.38 However, the NO2 adsorption

on magnetic A-MoS2 results in a reduction in total spin moment from 2 to 1

. This reduced

magnetic moment is attributed to the displacement of the antisite atom from its original position toward the center of nearest Mo-Mo bond. In this case the charge transfer (0.86 e) is observed from the gas molecule to the A-MoS2. The spatial charge density distribution (Fig. 3 c-d) shows that the majority of this charge density is localized at the antisite defect. This huge charge transfer results in a stronger binding of the gas molecule to the MoS2 substrate which is reflected from its lower adsorption energy, i.e., -2.57 eV. Similar to NO2, the adsorption of paramagnetic molecule NO also leads to the displacement of antisite atom from its original position towards the nearest hexagonal position. The NO molecule is found to orient at an angle 48o with respect to the 6


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monolayer surface with their center of mass located on the top of Mos-Mo bond. This structure also results in a reduced spin moment of 1

. For NO gas, the adsorption energy is found to be

the lowest, i.e., -2.94 eV in comparison to the adsorption of other gas molecules. Like NO2, the adsorption of NO also results in a large charge transfer (0.83 e) between the gas molecule and AMoS2. It is worthwhile to mention that, the NO gas may also be adsorbed perpendicular to the AMoS2 plane with its center of mass located directly on the top of antisite atom (Fig S1 in the Supporting Information). This configuration can result in a magnetic moment of 3

. However,

this structure has higher energy and therefore is unstable. The diatomic molecules CO has a similar adsorption configuration as NO with the orientation angle 51o with respect to the monolayer surface. The adsorption energy and charge transfer are found be -1.55 eV and 0.35 e, respectively. Like NO, CO can also exhibit a perpendicular configuration which is also unstable. The CO2 gas molecule is preferentially adsorbed perpendicular to the monolayer surface with its center of mass exactly on top of antisite atom. The adsorption energy and charge transfer for CO2 are -0.34 eV and 0.02 e, respectively, which are smaller but still significant. The other possible configuration of CO2 is found to be unstable (Fig S1 in the Supporting Information). It is interesting to note that the adsorption induced huge charge transfer between A-MoS2/gas will ease the detection of gas molecules by fabricating chemiresistors.39 To further explore the electronic interaction between the adsorbed gas species and A-MoS2, we investigate the electronic density of states (DOS) and projected density of state (PDOS. From Fig 5, it is noticed that the electronic interaction between the antisite atom and the gas molecules is highly sensitive to the adsorption configuration. For gas molecules with perpendicular adsorption configuration, such as; NH3 and CO2, a stronger interaction between gas molecules (more precisely,

-orbital of the

-orbital of the gas atom nearest to the antisite atom) and

orbital of antisite atom is observed. In case of NH3 the hybridization is dominated by NMo-

, whereas, for CO2 the hybridization is dominated by O-

and Mo-

and

orbitals. For the

gas molecules with a tilted adsorption configuration, such as; NO2, NO and CO, a stronger hybridization is observed between Mo-

,

(

and gas molecules p-orbitals. For

examples, for both NO2 and NO a strong hybridization of N-p, O-p orbitals with Moobserved, whereas, for CO, the hybridization is dominated by C-p, O-p and Mo7


is

orbitals.


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Moreover, the relative shifts in mid gap defect states (indicated by arrows) for different gas molecules are in good agreement with the magnitude of charge exchanged between the gas molecule and A-MoS2. For example, the adsorption of CO, NO and NO2 on A-MoS2 lead to a relative increase in the charge transfer (see Table 1), which is reflected as systematic increase in shifting of

state (shown in red arrows in Fig. 5) from its original position (as observed in A-

MoS2). Similar analogy can also be drawn for the adsorption of NH3 and CO2 on A-MoS2. It is worthwhile to mention that, similar to adsorption of NO and NO2, the adsorption of CO on A-MoS2 not only leads to the displacement of antisite atom from its original position, but also leads to the appreciable charge transfer (0.35 e). However, the adsorption of NO and NO2 reduces the spin moment to 1

, whereas the adsorption of CO does not affect the total spin moment of

A-MOS2. The reduction in magnetic moment observed for adsorption of NO and NO2, is attributable to the generation and partial occupancy of minority spin states below the Fermi level (Fig. S2 in the Supporting Information). In contrast, the minority spin states generated due to the adsorption of CO remain unoccupied and thus does not affect the total spin moment of A-MoS2 (Fig. S2 in the Supporting Information). Strain modulation of gas adsorption in A-MoS2 To explore the effect of strain, we first examined the adsorption of gas molecules on A-MoS2 at two strained states (-7% and 7%) and compared the results without strain, as shown in Fig. 6. It is noted that the adsorption energy of NH3 decreases significantly on application of the tensile strain, whereas the adsorption energy of CO and CO2 are slightly lowered with the tensile strain. For the gas molecules of NO2 and NO, the adsorption energies increase under both the compressive and tensile strain. This contrasting behavior of the gas molecules towards strain can be attributed to the adsorption configuration (Fig. S3 in the Supporting Information). In addition, the adsorption of NO and NO2 maintains the constant spin moment (1

) under both compressive and tensile

strain, whereas there exists a switching of magnetic states from a high spin state to the low spin state under the tensile strain for the adsorption of NH3, CO, and CO2. This switching property descend from the A-MoS2 without adsorbed gas species (discussed later). From these results, it can be understood that strained A-MoS2 can be a novel avenue for the NH3 gas sensor. Thus, herein we put our emphasis on exploring the underlying physical mechanism for the strain modulation of adsorption of NH3 on A-MoS2. 8


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Fig. 7 illustrates the strain modulation of the adsorption energy for NH3 gas adsorbed on the A-MoS2 surface. It can be noted that the application of biaxial tensile strain leads to a gradual lowering of the adsorption energy from 0 to 4%. At a critical tensile strain of 5%, a sharp increase in sensitivity can be realized as sudden drop in adsorption energy. In sharp contrast, application of biaxial compressive strain gradually weakens the interaction between the NH3 and A-MoS2, which is reflected from the increase in adsorption energy under compressive strain. This drastic variation in adsorption energy can be ascribed to the change in bonding configuration (inset of Fig. 7). To have insight into the variation in bonding configuration between antisite atom and nearest gas atom, the MoS-N bond length is presented in Fig. 7. It is observed that the bond length follows the same trend as adsorption energy. Under compressive strain, a gradual increase in MoS-N bond length from 2.29 (0% strain) to 2.31 Å (-7% strain), supports the increase in the adsorption energy. However, a nonlinear decrease in MoS-N bond length under tensile strain reveals much stronger interaction between NH3 and antisite defect and also supports the lowering of adsorption energy. By comparing the strain sensitive NH3 adsorption on A-MoS2 with that on pristine MoS2 (see Ref. 34), it can be understood that the MoS2 can exhibit enhanced sensitivity towards NH3 adsorption only at very high strain (~ 12.5 %). At this high strain the NH3 gas has a detrimental effect on mechanical property (realized as structural breakage) of monolayer MoS2. However, MoS2 functionalized with antisite defect display superior sensitivity without disturbing the structural integrity. To further explore the interaction between gas molecule and A-MOS2, electronic density of states are analyzed in Fig. 8. As mentioned before, the NH3 gas molecule and A-MoS2 interact via MoS-

and N-pz orbital hybridization which can be realized from the overlapping of these two

orbitals above the Fermi level. A back and forth charge transfer mechanism can be easily realized by comparing the position of the defect states near the Fermi level, with and without NH3 adsorption. At zero strain, the small downshift of to antisite atom, whereas, a large upshift of

orbital indicates charge transfer from NH3

and a orbitals clarifies a relatively larger charge

transfer from antisite atom to the gas molecule (the origin of ,

and a are discussed in the next

section). This process establishes the net charge transfer from A-MoS2 to NH3. Similar analogy can be drawn for large compressive strain (-7%), where reduced charges transfer is envisaged between the A-MoS2 and NH3. Since the defect states disappears at high tensile strain (≥ 5%) (discussed later) and also the NH3 adsorption does not have substantial effect on Fermi level.28 9



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Therefore, it is difficult to identify the charge transfer mechanism from the density of states at very high tensile strain. In order to further understand the underlying physical mechanism for charge transfer between NH3 and A-MoS2, Bader charge analysis is carried out (Fig. 9). In strain free condition, the adsorption of NH3 on MoS2 yields net charge transfer from NH3 to MoS2.35 However, when NH3 is adsorbed at highly spin polarized antisite Mo (1.80 e), as a consequence a small amount of charge is back donated to NH3.40 This process yields a net charge transfer of 0.04 e from antisite atom to the gas molecule. From a compressive strain -7% to tensile strain 4% the net charge transfer is found to be from antisite atom to the gas molecule. However, for strain ≥5% the charge transfer process is reversed, i.e., the charge is transferred from NH3 to the A-MoS2. The reason being, the charge density on antisite atom disappears at 5% and A-MoS2 behaves as charge acceptor instead of charge donor. The charge transfer from the NH3 to MoS2 at higher strain (7%) can be observed as an accumulation of charge density on A-MoS2 surface (indicated by arrow). Furthermore, the application of strain to NH3 adsorbed A-MoS2 results in switching of magnetic states. This switching characteristic stems from the switching of magnetic state in A-MoS2, discussed in the next section. Thus, from the above result it can be concluded that the antisite doped MoS2 can be a novel avenue to design highly sensitive and selective ammonia gas sensor. Strain Modulation of Magnetic States in A-MoS2 The evolution of surface spin moment as a function of biaxial strain is illustrated in Fig. 10(a). It is noticed that the total spin moment due to antisite doping is 2 (2

. This surface spin moment

) in A-MoS2 is stable from a compressive strain as large as -7% to a tensile strain 4%. For

tensile strain ≥ 5% the spin moment drops to zero. In order to explore this intriguing modulation of spin moment, the variation of local bonding configuration around the defect site is illustrated in Fig. 10 (b). Here, we define two parameters, h and θ, where h is the height of the antisite atom from the upper S-surface (inset Fig. 10-b) and θ denotes the angle Mo-Mo-S. It is noticed that, in unstrained condition the antisite atom moves slightly in out-of-plane direction and its height from the upper S-surface is 0.24 Å (assuming atoms to be point objects). Under compressive strain, the out of plane displacement of antisite Mo atom further increases to 0.35 Å for -7% strain. However, on application of tensile strain antisite atom gradually moves into the MoS2 plane. Surprisingly, for a critical strain of 5% the antisite atom suddenly jumps below the S-plane (here, zero in the left 10


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hand scale indicates S-plane) by a height 0.60 Å. Henceforth, the height of antisite atom remains unchanged with increase in tensile strain. Furthermore, the parameter θ distinguishes MoS2 from atomic layer thick graphene. Graphene under strain undergoes elastic deformation which involves change in the in-plane C-C bond length and C-C-C bond angle. In contrast, MoS2 under compression or tension leads to an out-of-plane displacement of upper and lower S-plane which changes S-Mo-S bond angle. For undoped MoS2 this bond angle is 81.2o, which is in good agreement with previous report.41 However, when one S-atom is replaced by Mo (antisite doping), the bond angle changes to 84.9o. It is noticed that under compressive strain the value of θ increases due to opposite displacement of antisite Mo and S atoms from middle Mo layer (inset, Fig. 2-b)). Under tensile strain the upper and lower S-planes come closer due to Poisson’s contraction which is also the reason behind the decrease in θ.42 For a critical strain of 5%, the value of θ suddenly drops to 50.8o. Interestingly, these changes in local geometry (h and θ) resembles with the change in magnetization discussed above, suggesting that the position of antisite atom controls the spin moment. To further explore the structural stability at large compressive and tensile strain (±7%), we carried out molecular dynamics simulations at room temperature (300 K), at a time step of 1 fs. It has been observed that even after 1000 steps the geometry of antisite doped MoS2 remains unchanged (for both ±7% strain), indicating that A-MoS2 is stable under large strain (Fig S4 in the supporting information). To gain insight into the intriguing modulation of magnetism by strain, the spatial spin density distribution

↑

↓

of a fully relaxed A-MoS2 is presented in Fig. 11-(a). It is seen that the spin-

polarized electrons are mainly localized at the antisite Mo atom and feebly distributed among the second nearest Mo atoms. The S-6 ring surrounding the antisite Mo atom is also slightly contributing the spin moment. For the quantitative estimation of localized and distributed spin moments a detail Bader analysis is carried out. The magnetic moment localized at the Mo atom is estimated to be 1.80

, which is 90% of the total magnetic moment. Fig. 11 (d) presents a

schematic diagram showing the possible orbital hybridization between antisite Mo and nearest and second nearest neighboring Mo atoms. The calculation results also show that the spin moment on antisite Mo gradually decreases from 1.80 to 1.53

(Fig. 10-a) with increase in compressive

strain from 0 to -7%, without affecting the total magnetic moment. In contrast, an increase in local spin moment from 1.80 to 1.83

is observed at antisite atom from 0 to 4% tensile strain. For

strain values ≥ 5%, the spin moment on the antisite Mo atom and on other atoms drops to zero. 11



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This drastic drop in spin moment can be witnessed from the disappearance of spin density [Fig.11(c)]. Moreover, for strain values ≥ 5%, the disappearance of spin density together with zero spin moment establishes nonmagnetic behavior. To further illuminate our understanding on strain modulated magnetization, an electronic structure insight has been presented in Fig. 12. It is noticeable that the application of small tensile strain (≤4%) does not affect the position of the in gap defect states. However, the conduction band edge and valence band edge are found to be susceptible to the tensile strain. It can be noted that both the edges comes closer to the Fermi level with increase in tensile strain. For a critical strain of 5%, the spin polarized defect states disappear and a true semiconducting behavior is witnessed for A-MoS2. This drastic variation electronic structure can be ascribed to the change in the hybridization (Fig. S5 in the Supporting Information), where the overlapping of d-orbital along with s-orbital indicates a low spin configuration. Thus, the disappearance of spin polarized defect states not only supports the nonmagnetic behavior observed for strain ≥ 5%, but also hints a transition from high spin to low spin configuration. In contrast, the application of compressive strain does not have substantial effect on the density of states near the Fermi level. Alternatively, the electronic properties of A-MoS2 is preserved under the application of compressive strain. This strain control of spin states observed in A-MoS2 will be of great significance to design new generation spintronic devices. Summary By using first principles calculations based on density functional theory, we demonstrate that the gas adsorption properties of monolayer-MoS2 can be significantly enhanced by MoS antisite doping and strain. It is found that the antisite defect doped MoS2 exhibit highly enhanced sensitivity towards the detection of chemical species such as NH3, NO2, NO, CO and CO2, in comparison to its undoped counterpart. The structural configuration, Bader charge analysis and electronic level insight have been implemented to explore the underlying physical mechanism. The unprecedented adsorption arises from concerted charge transfer between the adsorbate and AMoS2. Among the studied gas species, the adsorption of NH3 is highly dependent on the biaxial tensile strain, whereas the adsorption of CO and CO2 exhibits weak strain dependence. In addition, the magnetic states of A-MoS2 and NH3, CO and CO2 adsorbed A-MOS2 can be switched by the

12


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application of in-plane tensile strain. These findings will pave a new path for fabricating devices for a diverse range of applications including gas sensors and spintronic devices. Supporting Information: 

High energy configurations for NO, CO and CO2 gas adsorbed on A-MoS2 (Fig S1), the DOS of A-MOS2 adsorbed with NO2, NO and CO gas molecules (Fig S2), variation in adsorption configuration of NO2, NO, CO and CO2 gas molecules on A-MoS2 at two different strains (±7 %) (Fig S3), snapshots of structural configuration of A-MoS2 taken at different time steps for two different strains (±7 %), by using molecular dynamics simulations (Fig S4), and DOS and PDOS of A-MoS2 at 5% tensile strain (Fig S5).

Acknowledgement The authors acknowledge financial support for M.P.K.S. and J.W. from the National Natural Science Foundation of China (Grant No. 11321202, 11472242), and T.S. and T.K. from JSPS KAKENHI Grant Number 25000012, 26289006 and 15K13831. References 1. Wu, M. T.; Fan, J. W.; Chen, K. T.; Chang, S. T.; Lin, C. Y., Band Structure and Effective Mass in Monolayer MoS2. J. Nanosci. Nanotechnol. 2015, 15 (11), 9151-7. 2. Ko, P. J.; Abderrahmane, A.; Thu, T. V.; Ortega, D.; Takamura, T.; Sandhu, A., Laser Power Dependent Optical Properties of Mono- and Few-Layer MoS2. J. Nanosci. Nanotechnol. 2015, 15 (9), 6843-6. 3. Le, C. T.; Senthilkumar, V.; Kim, Y. S., Photosensitivity Study of Metal-SemiconductorMetal Photodetector Based on Chemical Vapor Deposited Monolayer MoS2. J. Nanosci. Nanotechnol. 2015, 15 (10), 8133-8. 4. Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M., Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16 (2), 1097–1103. 5. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6 (3), 147-50. 6. Yu, Z.; Ong, Z. Y.; Pan, Y.; Cui, Y.; Xin, R.; Shi, Y.; Wang, B.; Wu, Y.; Chen, T.; Zhang, Y. W.; Zhang, G.; Wang, X., Transistors: Realization of Room-Temperature PhononLimited Carrier Transport in Monolayer MoS2 by Dielectric and Carrier Screening, Adv. Mater. 2016, 28 (3), 546. 7. Ponomarev, E.; Gutierrez-Lezama, I.; Ubrig, N.; Morpurgo, A. F., Ambipolar LightEmitting Transistors on Chemical Vapor Deposited Monolayer MoS2. Nano Lett. 2015, 15 (12), 8289-94. 8. Radisavljevic, B.; Whitwick, M. B.; Kis, A., Integrated Circuits and Logic Operations based on Single-Layer MoS2. ACS nano. 2011, 5 (12), 9934-8.

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Table I: Calculated values of adsorption distance (dads), charge transfer (CT), and magnetic moment (M) when gas molecule is adsorbed on the surface of MoS2 and A-MoS2.

Gas Molecule

dads-MoS2 (Å)

dadsA-MoS2 (Å)

CT MoS2 (e)

CT A-MoS2 (e)

M( ) MoS2

M( ) A-MoS2

NH3

2.94

2.29

-0.02*

0.04

0

2

NO2

3.18

2.40

0.06

-0.86*

1

1

NO

3.30

2.45

0.02

-0.83*

1

1

CO

3.58

2.65

0.006

-0.35*

0

2

CO2

4.22

3.53

-0.001*

0.02

0

2

*Negative CT indicates the charge transfer from the gas molecule to A-MoS2.

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Table of Contents (TOC) Image

17



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Fig 1: (a-b) Top and side view of pristine MoS2 and (c-d) MoS antisite doped MoS2. The orange, pink and light blue balls represent Mo- atom, S-atom and MoS antisite defect, respectively.


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Fig 2: Density of states (DOS) and projected density of states (PDOS) of monolayer MoS2 doped with MoS antisite defect.



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NH3

NO2

NO

CO

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CO2

(a)

(b)

(c)

(d)

Fig 3: (a-b) Top and side view of most stable adsorption configuration of NH3, NO2, NO, CO and CO2 on the A-MoS2 surface, (c-d) the charge density difference plots for NH3, NO2, NO, CO and CO2 interacting with A-MoS2. The green (red) isosurface indicated the charge accumulation (depletion). The isosurface value is taken to be 0.001 eV/ Å.


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Fig. 4: Adsorption energy for pristine MoS2 and A-MoS2 adsorbed with NH3, NO2, NO, CO and CO2 gas molecules. For adsorption of gas molecules on A-MoS2, the adsorption energy decreases by two orders of magnitude in comparison that on pristine MoS2.



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Fig 5: Density of states (DOS) and projected density of states (PDOS) of A-MoS2 adsorbed with NH3, NO2, NO, CO and CO2 gas molecules. The DOS of A-MoS2 is shown in the top to understand the relative shift in the defect states. The relative shift in 𝑒1′ , 𝑒1′′ and a states are shown in red, olive green and black arrows.


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Fig. 6: Adsorption energy and magnetic moment for A-MoS2 adsorbed with NH3, NO2, NO, CO and CO2 gas molecules at -7%, 0% and 7% strain.



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Fig. 7: Variation of adsorption energy and bond length (MoS-N) as a function of strain for NH3 adsorbed on A-MoS2. Inset shows the structural configuration of NH3 adsorbed A-MoS2 at compressive (-7%) and tensile (7%) strain.


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Fig. 8: DOS of strain free and strained NH3 adsorbed A-MoS2. For comparison, DOS of A-MoS2 is presented on the top. The red arrows indicate the directions of shifting of the defect states.



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Fig. 9: Variation of charge transfer and magnetic moment as a function of in-plane biaxial strain for NH3 adsorbed A-MoS2.


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Fig. 10: (a) Variation in total magnetic moment in antisite doped MoS2 and magnetic moment on antisite Mo atom with respect to the applied strain, (b) variation in height of antisite Mo atom from upper S-surface and Mo-Mo-S bond angle with respect to applied strain. Inset of (a) shows the structural configuration of A-MoS2 at different strains, and inset (b) elucidates the change in the Mo-Mo-S bond angle at -7, 4, 5 and 7 % strain and a schematic showing the position of MoS defect on the surface of MoS2.



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(a)

(b)

e-

(d)

e+

(c)

e+

-

e

e-

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+

e

e-

Fig. 11: Spatial distribution of spin density in antisite 𝑀𝑜𝑆 decorated MoS2 at strain 0% (a), -7% (b), and 5% (c). The green and yellow isosurfaces indicate spin up and spin down component (isosurface value was set to 0.001 eV/Å). The antisite defect is indicated by arrow. (d) schematic diagram showing the possible orbital hybridization between ansite defect (center of the triangle) and nearest neighboring Mo atoms.


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Fig. 12: Density of states of A-MoS2 under different compressive and tensile strain (-7% to 7%).


Modulation of Gas Adsorption and Magnetic Properties of Monolayer

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