Tuning of electronic states and magnetic polarization

in monolayered MoS2 by codoping with transition

metals and nonmetals

Yaping Miao1,2 , Yuhong Huang3, Qinglong Fang1, Zhi Yang1, Kewei Xu1,4, Fei Ma1,2,*, and Paul K. Chu2,*

1State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China2Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China3College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, Shaanxi, China4Department of Physics and Opt-electronic Engineering, Xi’an University of Arts and Science, Xi’an 710065, Shaanxi, China

Received: 24 May 2016

Accepted: 4 July 2016

Published online:

15 July 2016

! Springer Science+Business

Media New York 2016

ABSTRACT

Doping is an effective way to modulate the properties of two-dimensional (2D)materials to cater to new applications. In this work, the codoping effects oftransition metals (TM) and nonmetals (NM) on the electronic states and mag-netic polarization in monolayered MoS2 are investigated by first-principlescalculation. The NM-doped MoS2 possesses semiconducting characteristics, butthe B-, N-, and F-doped ones are magnetic showing a magnetic moment of 1.0 lBper dopant. The metal dopants can induce magnetization in the monolayeredMoS2, and the magnetic moment is related to the total number of outer electronsin the TM. In the case of codoping, the S and Mo atoms are substituted by theNM and TM atoms, respectively, and hence, the semiconducting characteristicsare maintained despite the reduced band gap. Spin polarization resulting fromcodoping depends on the number of outer electrons in the TM and NM dopants.Spin polarization occurs if the total number of the outer electrons is odd, butdoes not if it is even. The magnetic moment of the codoped monolayered MoS2is always 1 lB and magnetism is enhanced considerably by (V ? B) and(Mn ? F) codoping.

Introduction

Isolation of graphene in 2004 [1] has spurredtremendous interest in two-dimensional (2D) mate-rials such as hexagonal boron nitride [2–4], graphene[5–7], silicene [8–10], transition metal dichalcogenides[11–14], and so on. There are more than 40

compounds in the TMD family and their electronicstructures depend on the coordination around thetransition metals as well as electron density in thenonbonding d bands [15, 16]. Consequently, TMDspossess physical properties that vary from insulatingto superconducting and have many potential appli-cations in nanoelectronics [17], photovoltaics [18],

Address correspondence to E-mail: mafei@mail.xjtu.edu.cn; paul.chu@cityu.edu.hk

DOI 10.1007/s10853-016-0195-y

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photocatalysis [19], photodetection [20], lithium ionbatteries [21], and nanoelectromechanical systems(NEMS) [22]. MoS2 is one of the typical TMDs withthe common formula of MX2 [23, 24], in which Mrepresents a transition metal and X is a chalcogen.MoS2 has a sandwiched configuration of covalentlybonded S–Mo–S hexagonal planes, which are bondedtogether by weak van der Waals forces in the bulk. Asa result, MoS2 can be exfoliated into ultrathin layersand impurities can be intercalated to tailor the elec-tronic states and physical properties.

Monolayered MoS2 can be fabricated by mechani-cal exfoliation and chemical vapor deposition (CVD)[25, 26]. Although bulk MoS2 is an indirect bandgapsemiconductor with a band gap of 1.3 eV, monolay-ered MoS2 has a direct band gap of 1.8 eV [27, 28].Many chemical and physical approaches such asdoping have been proposed to manipulate the elec-tronic states and magnetic properties of 2H-MoS2. Forexample, Yue et al. have functionalized monolayeredMoS2 by substitutional doping and found that H, B,N, and F can induce local magnetic moments [29],and Lin et al. have studied the electronic and mag-netic properties of MoS2 doped with Mn, Fe, and Coand found that they are all magnetic except for thehighest positive charged states [30]. From the theo-retical aspect, Cheng et al. have conducted first-principles calculation to study the electronic andmagnetic properties of monolayered MoS2 dopedwith transition metals. Magnetism is observed fromthe Mn-, Fe-, Co-, Zn-, Cd-, and Hg-doped materialsbut suppressed in samples doped with other ele-ments due to Jahn–Teller distortion [31]. Experimentshave also been performed to investigate magnetismin monolayered MoS2 [18, 32–35]. Komsa et al. havefilled the vacancies in monolayered MoS2 by substi-tutional impurities and the physical properties ofMoS2 can be tuned [14]. Similar doping schemes havebeen implemented on other 2D materials such ascarbon nanotubes [36, 37], graphene [6, 38], andhexagonal boron nitride [39, 40]. Gai et al. haveobserved shifts in the valence band edge of TiO2 dueto (Mo ? C) codoping [41], and Cheney et al. havedemonstrated that Cr–N codoping enhances theelectronic states and reduces the band gap of TiO2

[42]. Furthermore, a strong dopant–dopant couplingbehavior is observed from TiO2 by density-functionalcalculation [43]. However, the coupling effect of TMand NM dopants in monolayered MoS2 is stillunknown, and herein, the codoping effects on the

electronic states and magnetic polarization areinvestigated.

First-principles calculation

Figure 1a displays the atomic model of monolayeredMoS2 with the hexagonal lattice and lattice constantof a = b = 3.16 A [44, 45], angle a = b = 90",c = 120", and space group of P63/mmc. The4 9 4 9 1 MoS2 supercell 12.64 A 9 12.64 A in size isadopted in the calculation, in which 32 S and 16 Moatoms are involved. Two types of impurities areintroduced into the supercell: (1) S atoms are substi-tuted by NM atoms, which is denoted by NMS

(Fig. 1b) and (2) Mo atoms are replaced by TM atoms,which is designated as TMMo (Fig. 1c). Figure 1dillustrates the codoping models, and three codopingconfigurations are taken into account: (1) the dopedTM and NM atoms are the nearest neighbors, (2) thedoped TM and NM atoms are the second-nearestneighbors, and (3) the doped TM and NM atoms arethe third-nearest neighbors. According to the calcu-lated formation energies, the configuration with thenearest neighboring TM and NM dopants is the moststable one owing to the interaction between them[46, 47]. Therefore, only this codoping configurationis considered in the following calculations. When oneMo/S atom in each supercell is substituted by a TM/NM atom, the doping concentration is 2.08 %. Whenone Mo and one S atom in each supercell are replacedby TM and NM atoms simultaneously, the overalldoping concentration is 4.17 %.

The calculation is based on the spin density-func-tional theory (DFT) within the framework of gener-alized gradient approximation (GGA), asimplemented in the Vienna ab initio simulationpackage (VASP) [48]. The core electron interaction isdescribed by the projector augmented wave (PAW)potential [49]. The Perdew–Burke–Ernzerhof (PBE)scheme is adopted in the exchange–correlationinteraction based on total energy pseudopotentialplane-wave method [50]. The atomic positions arefully optimized before single-point calculation, and akinetic energy cutoff of 480 eV is used in the simu-lation for the plane-wave basis set. Conjugate gradi-ent minimization is used for all structural relaxationuntil the force on each atom is less than 0.01 eV/Aand the energy change per atom is less than1.0 9 10-5 eV. A 15 A vacuum is added to the

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monolayer to avoid interlayer coupling and7 9 7 9 1 and 9 9 9 9 1 k-point meshes areemployed in geometrical optimization and totalenergy calculation, respectively.

Results and discussion

Structure stability

The optimized geometrical structure shows that dif-ferent dopants will induce distinct lattice distortion.To evaluate the relative stability of the monolayeredMoS2 as well as TM- and NM-doped ones, the for-mation energy Eform is calculated as follows:

Eform ! Edoped"Epure" n lM"lMo# $ #1$

Eform ! Edoped"Epure"n lX"lS# $; #2$

in which Edoped is the total energy of the relaxed4 9 4 9 1 MoS2 supercell with one Mo (S) atomsubstituted by TM (NM) atom, Epure is the totalenergy of the optimized 4 9 4 9 1 MoS2 supercell,and lM and lX are the chemical potential of thedoped TM and NM atoms, respectively. Table 1 liststhe calculated formation energies of the dopants. Thesmaller the formation energy, the more stable is the

doped configuration. Table 1 shows that the forma-tion energies of B-, O-, F-, V-, Cr-, Mn-, and Fe-dopedmonolayered MoS2 are negative and so they arestable. The formation energy of B-doped MoS2 is thesmallest, indicating that B doping is favorable. Withregard to TM doping, the formation energies of Co,Ni, and Cu are high so that doping is difficult,although Co has been doped at the edges of MoS2sheets by sulfidation with a mixture of ammoniumheptamolybdate and cobalt nitrate [32]. Therefore, itis expected that a similar chemical approach can beapplied to other substitutional TM dopants inmonolayered MoS2. The buckled height between theMo and S atoms in the pristine MoS2 is 1.56 A. Theinterlayered Mo and NM atoms are covalently bon-ded with each other and more strongly bonded thanMo–S in pristine MoS2 (Table 1). On the other hand,since the radius of the TM atom is smaller than that ofMo and the separation between TM and S is shorterthan that of Mo and S in pristine MoS2, interlayercoupling between TM and S atom is strong.

Electronic states and magnetization

The total density of states (TDOS) and partial densityof states (PDOS) of doped MoS2 are calculated to

Figure 1 Top view of the 4 9 49 1 supercell of the pristine anddoped monolayered MoS2: a Pristine monolayer MoS2, b NMdoping at the S site, c TM doping at the Mo site, d Codoping ofNM and TM. Three codoping configurations are taken into accountin this work: (1) the doped TM and NM atoms are the nearest

neighbors, (2) the doped TM and NM atoms are the second-nearestneighbors, and (3) the doped TM and NM atoms are the third-nearest neighbors. The blue, orange, red, and green balls denoteMo, S, NM, and TM atoms, respectively.

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study the influence of TM/NM doping on magneticpolarization. Figure 2a shows the TDOS and PDOS ofthe pristine monolayered MoS2 revealing semicon-ducting characteristics with a band gap of 1.8 eV. Themagnified PDOS near the VBM and CBM of thepristine monolayered MoS2 is plotted in the inset ofFig. 2a. The PDOS of Mo s, p, d orbitals and S s, porbitals could be clearly identified. It can be foundthat the states near the VBM are dominated by Mo dand S p orbitals, whereas those near the CBM aremainly composed of Mo d orbital, as predicted pre-viously [47, 51]. Figure 2b–f depicts the TDOS andPDOS of the B-, C-, N-, O-, and F-doped MoS2,respectively. As shown in Fig. 2b, c, the TDOS is stillsymmetrical, indicating that C and O cannot inducemagnetization. The C- and O-doped monolayeredMoS2 are semiconducting with band gaps of 0.9 and1.73 eV, respectively. Taking into account the elec-tronic configurations of C 2s22p2 and S 3s23p4, it isequivalent to inducing two holes when each S atom issubstituted by one C atom resulting in deep impuritylevels in the middle of the gap (Fig. 2b) [52]. O(2s22p4) and S (3s23p4) have the same number ofouter electrons and so the TDOS and PDOS ofO-doped MoS2 change slightly (Fig. 2c). Only thestates in VBM are more localized than that in thepristine MoS2 (Fig. 2a). On the contrary, as shown inFig. 2d–f, the PDOS of the spin-up and spin-downchannels are asymmetrical, indicating that B, N, andF can induce magnetization. B incorporation leads top-type characteristics with a band gap of 0.14 eVsince the impurity levels are near the valence band(Fig. 2d). As shown in Fig. 2e, the impurity levels inN-doped MoS2 are very close to the VBM. Althoughall the states in the majority-spin channel are

occupied, those in the minority-spin channel arepartially occupied and some are above the Fermilevel and N incorporation renders the MoS2, a semi-metal. Moreover, the sharp peak near the Fermi levelindicates strong hybridization between Mo and N. Asfor F doping (Fig. 2f), the Fermi level shifts up com-pared to the pristine monolayered MoS2, and theimpurity levels are near the CBM being characteristicof n-type. Substitutional B, N, and F can inducemagnetization as predicted previously in [29]. Thefirst-principles calculation also illustrates that thespin-polarized state is more stable than the nonspin-polarized one (Table 1). The total magnetic momentof the B-, N-, and F-doped monolayered MoS2 is1.0 lB, but the contributions from B, N, and F are only4.9, 21.2, and 6.2 %, respectively. The magneticmoment of the NM-doped MoS2 is related to theparity of the number of the outer electrons in NMatom. If the number of the outer electrons in NMatoms such as C 2s22p2 and O 2s22p4 is even, thematerials are nonmagnetic. However, magnetism isobserved if the number of the outer electrons in theNM atoms such as B 2s22p1, N 2s22p3, and F 2s22p5 isodd due to the unpaired electrons in the substitu-tional B/N/F atoms in the monolayered MoS2.

Similar calculation is performed with Mo replacedby the TM atom. Substitutional V, Mn, Fe, Co, Ni, andCu can induce magnetization, but substitutional Crcannot [53]. TM doping induces a magnetic momentequal to the difference in the number of outer elec-trons between TM and Mo atoms. For example, themagnetic moment is 1 lB if Mo (4d55s1) is substitutedby Mn (3d54s2) and is 4 lB if Mo (4d55s1) is substi-tuted by Ni (3d84s2). As shown in Table 1, the for-mation energy of TM-doped monolayered MoS2

Table 1 Distance between thedopant and Mo/S atoms (d),forming energy (Eform),magnetic moment persupercell (ltotal), magneticmoment per dopant (l), andenergy difference between spinand nonspin states (De)

System dM-X (A) Eform (eV) ltotal (lB) l (lB) Eg (eV) De (eV)

Mo16S31B1 0.951 -2.139 1 0.055 0.14 -0.040Mo16S31C1 0.880 3.527 0 0 0.9 0Mo16S31N1 0.991 4.044 1 0.19 1.68 -0.059Mo16S31O1 1.102 -1.374 0 0 1.73 0Mo16S31F1 1.368 -1.664 1 0.06 0.44 -0.173Mo15V1S32 2.048 -3.004 1 0.281 0 -0.012Mo15Cr1S32 2.036 -2.396 0 0 1.5 0Mo15Mn1S32 2.016 -0.862 1 0.937 0 -0.134Mo15Fe1S32 1.976 -0.059 2 1.119 0.19 -0.151Mo15Co1S32 2.058 1.136 3 1.162 0 -0.172Mo15Ni1S32 2.071 1.879 4 1.255 0 -0.361Mo15Cu1S32 2.076 3.043 5 1.541 0.15 -0.490

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changes from -3.004 to 3.043 eV. The V-, Cr-, andMn-doped samples are more stable than those withother TM dopants. Hence, these three doping con-figurations are mainly considered in the followingwork. Figure 3a–c shows the TDOS and PDOS of V-,Cr-, and Mn-doped MoS2. Since V (3d34s2) has oneless d-electron than Mo (4d55s1), substitutional Vinduces a hole in MoS2. As shown in Fig. 3a, theV-doped sample shows defect states across the Fermilevel. It is due to hybridization between V and Mo asevidenced by the overlapping PDOS in the energyrange between -0.2 and 0.2 eV in the bottom panel ofFig. 3a. The V 3d and Mo 4d states are also

hybridized in the energy range of 1.6–1.85 eV nearthe CBM. The calculated magnetic moment of theV-doped MoS2 is *1 lB per supercell, which is con-sistent with reported results [53]. Figure 3b shows theTDOS and PDOS after Cr in corporation. The PDOSof the majority-spin and minority-spin channels aresymmetrical indicative of nonmagnetic features,because Cr (3d54s1) and Mo (4d55s1) have the samenumber of outer electrons. The impurity statesaround the CBM are mainly composed of Cr 3d and S3p reducing the band gap to 1.5 eV. As shown inFig. 3c, the Mn-doped MoS2 is a semimetal[30, 53, 54]. There are occupied and unoccupied states

Figure 2 a TDOS and PDOS of the pristine monolayered MoS2.The magnified PDOS near the VBM and CBM of the pristinemonolayered MoS2 is plotted in the inset. b–f C-, O-, B-, N-, andF-doped monolayered MoS2. The upper panel is TDOS and the

bottom panel is the PDOS of the p orbital of the NM atom, dorbital of Mo, and p orbital of S. The defect states are marked byviolet dotted rectangles.

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near the Fermi level in the majority-spin channel andthe Fermi level is pushed far away from the VBM. Onthe other hand, the impurity states in the minority-spin state are pushed near the CBM. The impuritystates at VBM are composed of Mo 4d and Mn 3dstates, whereas hybridization between Mn 3d and S3p states produces an unoccupied defect level belowthe CBM by about 0.2 eV (Fig. 3c).

NM/TM doping can tune the electronic states andmagnetic polarization in monolayered MoS2 onaccount of the interactions between dopants andMoS2. In fact, if the dopants are close to each other,coupling between them is substantial. Hence, codop-ing with TM and NM is also studied to provide more

insights into the general doping effect. Figure 4ashows the optimized (Cr ? X) codoped MoS2 in whichX stands for the NM atoms. Upon relaxation, the NMatom moves downwards by 47.7, 55.4, 34.9, 31.1, and2.69 %, respectively, resulting in reduced lattice sym-metry. The separation between two S layers is alsoreduced compared to that in pristine MoS2. Figure 4b–c displays the TDOS and PDOS after (Cr ? C) and(Cr ? O) codoping. Both exhibit nonmagnetic statessimilar to the C- and O-doped ones. The impurityenergy levels near the CBM are mainly contributed bythe Cr 3d and C 2p orbitals (Fig. 4b). The (Cr ? C)codoped sample is still a semiconductor with a bandgap of 1.3 eV. The conduction band is dispersed andthe electron mobility is expected to be rather large [46].In the (Cr ? O) codoped system, there are alsoimpurity levels as a result of hybridization betweenthe Cr 3d and S 3p orbitals. The band gap after(Cr ? O) incorporation is reduced to 1.4 eV. As shownin Fig. 4d–f, (Cr ? B), (Cr ? N), and (Cr ? F) codop-ing induces magnetization in the monolayered MoS2similar to B, N, and F doping. However, there aredifferent electronic states in the (Cr ? B), (Cr ? N),and (Cr ? F) codoped samples. For example,hybridization between the Cr 3d and B 2p orbitals isobserved at the CBM from the (Cr ? B) codoped MoS2(Fig. 4d) and is significantly stronger than that in theB-doped one (Fig. 2b). Although no remarkablehybridization between Cr and N atom is observed, theN 2p orbital is split into two states and stronglyhybridizes with the 4d orbital of adjacent Mo atoms(Fig. 4e). The impurity levels in the (Cr ? N) codopedMoS2 in the majority-spin channel are completelyoccupied, whereas those in the minority-spin channelare pushed away from the Fermi level. Figure 4fshows the TDOS and PDOS of the (Cr ? F) codopedmonolayered MoS2 revealing that the Fermi level isclose to the valance band as well as stronghybridization between the Cr 3d and S 3p states.Therefore, (Cr ? C) and (Cr ? O) codoping justaffects the electronic states, while (Cr ? B), (Cr ? N)and (Cr ? F) codoping not only affects the electronicstates but also induces magnetization in the mono-layered MoS2. Moreover, the magnetic moment of the(Cr ? B), (Cr ? N), and (Cr ? F) codoped samples isrelated to the parity of the number of the outer elec-trons in the NM atom. If it is odd (B 2s22p1, N 2s22p3,O 2s22p4), it is magnetic, but nonmagnetic if thenumber of the outer electrons in the NM atoms is even(C 2s22p2 and O 2s22p4).

Figure 3 TDOS and PDOS of a V-, b Cr- and c Mn-dopedmonolayered MoS2. The upper panel is TDOS, the bottom panel isPDOS of the d orbital of the TM atom, d orbital of Mo, and porbital of S. The defect states are marked by violet dottedrectangles.

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Figure 5a shows the optimized (V ? X) codopedmonolayered MoS2 in which X stands for the NMatoms. The buckled height between the top andbottom S layers in the (V ? C), (V ? N), (V ? O),and (V ? F) codoped samples is reduced comparedto the primitive MoS2, but it is enlarged in the(V ? B) codoped one. The separation between V andS is also reduced. Different from Cr (3d54s1), each V(3d34s2) atom has one less electron than Mo (4d55s1)atom. Figure 5b–c displays the TDOS and PDOSafter (V ? C) and (V ? O) codoping. Both are stillsemiconductors with a small band gap, but themajority-spin and minority-spin channels are splitresulting in magnetization and the impurity states

appear above the Fermi level in the minority-spinchannel. As shown in Fig. 5e–f, the (V ? N) and(V ? F) codoped samples are nonmagnetic becauseof the symmetrical PDOS of the majority-spin andminority-spin channels. It is different from theresults acquired after (Cr ? X) codoping (Fig. 4b–cand e–f). If the total number of the outer electrons inV and NM atoms is odd, (V ? X) codoping intro-duces a magnetic moment of 1 lB as shown inTable 2. On the other hand, if the total number of theouter electrons in V and NM atoms is even, non-magnetic properties are observed. For example,codoping with V (3d34s2) and C (2s22p2) results inmagnetism, but codoping with V and N (2s22p5)

Figure 4 a Optimized structure of the codoped monolayeredMoS2 and b–f TDOS and PDOS of the Cr–B, Cr–C, Cr–N, Cr–O,and Cr–F codoped samples. The upper panel is TDOS and the

bottom panel is PDOS of the d orbital of Cr, d orbital of Mo, and porbital of the NM and S. The defect states are marked by violetdotted rectangles.

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does not. Although the magnetic state of the(Cr ? X) codoped MoS2 is different from that of the(V ? X) codoped one, the former will become mag-netic if the total number of the outer electrons in Crand NM atoms is odd. In particular, the (V ? B)codoped system still has magnetism, although thetotal number of the outer electrons in V and B atomsis even. As indicated by the almost overlappingPDOS of V 3d and B 2p orbitals in the energy rangebetween -1.5 and 0.5 eV (Fig. 6a), there is a stronginteraction between the V 3d and B 2p orbitals.Figure 6b shows the charge density difference in the(V ? B) codoped sample, disclosing that the elec-trons concentrate on B and neighboring V, Mo1, and

Mo2 atoms. The interaction between V and B as wellas Mo and B is much stronger than that of pristineMo–S bonds, and the bond strength between V andB is larger than that between Mo and B. According tothe Bader charge analysis, 1.35 e on each V atom istransferred to the B atom, and 0.11 e is transferredfrom the two nearest neighboring Mo atoms to Batom. The charge density difference is symmetricalaround B and, consequently, (V ? B) codopingpromotes localization of nonbonding V 3d and B 2pgiving rise to a magnetic moment of 2 lB. The TDOSand PDOS in Fig. 5d also demonstrate that spinpolarization arises primarily from the d orbitals ofthe Mo and V atoms and p orbital of the B atom.

Figure 5 a Optimized structure of the codoped monolayeredMoS2 and b–f TDOS and PDOS of the V–B, V–C, V–N, V–O,and V–F codoped samples. The upper panel is TDOS and the

bottom panel is PDOS of the d orbital of V, d orbital of Mo, and porbital of the NM and S. The defect states are marked by violetdotted rectangles.

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(Mn ? B) and (Mn ? N) codoping produces non-magnetic properties, but (Mn ? C) and (Mn ? O)codoping introduces spin polarization featuring amagnetic moment of 1 lB (Table 2). If the total

number of the outer electrons in Mn and NM atoms isodd, the (Mn ? X) codoped sample becomes mag-netic. On the contrary, if it is even, nonmagneticproperties are observed and the formation energyand magnetic moments are listed in Table 2. All the(Mn ? X) codoped samples are still semiconductingas a result of the compensation effects rendered byMn and NM atom being the donor and acceptor,respectively. (Mn ? F) codoping yields similarresults as (V ? B) codoping. Although F is the mostelectronegative among the NM atoms, the interactionbetween Mn and F is not strong, as shown in thebottom panel of Fig. 6c, and hence, the charge densitydifference around the F atom decreases but thataround Mn and Mo increases (Fig. 6d). The interac-tion between Mn and F as well as Mo and F isweakened compared to the adjacent Mn–S and Mo–Sbonds. Each doped F atom captures 0.18 e from theadjacent Mo and Mn atoms. Figure 6d shows sub-stantial electron transfer from Mn to neighboring Satoms indicating strong bonding between them.Although (Mn ? F) codoping reduces the latticesymmetry, the interaction between neighboring Mo

Figure 6 a PDOS of the d orbital of V, p orbital of B andb differential electron density in the hexagonal plane of the Moatoms in the (V ? B) codoped monolayered MoS2. c d orbital ofMn, p orbital of F and d the differential electron density in the

hexagonal plane of the Mo atoms in the (Mn ? F) codopedmonolayered MoS2. The red and blue colors indicate accumulationand depletion of electrons, respectively, and the color scale is0.03 e/A3.

Table 2 Forming energy (Eform), magnetic moment per supercell(ltotal), magnetic moment per dopant (l), and band gap Eg

System Eform (eV) ltotal (lB) Eg (eV)

Mo15Cr1S31B1 -1.777 1 0.245Mo15Cr1S31C1 1.019 0 1.34Mo15Cr1S31N1 2.539 1 0.251Mo15Cr1S31O1 -3.601 0 1.45Mo15Cr1S31F1 -1.670 1 0.91Mo15V1S31B1 -1.927 2 0.17Mo15V1S31C1 3.845 1 0.18Mo15V1S31N1 1.964 0 0.43Mo15V1S31O1 -0.998 1 0.15Mo15V1S31F1 -2.732 0 1.51Mo15Mn1S31B1 -1.375 0 1.08Mo15Mn1S31C1 5.358 1 0.41Mo15Mn1S31N1 3.531 0 0.78Mo15Mn1S31O1 0.983 1 0.38Mo15Mn1S31F1 -0.604 2 0.52

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and S atoms is enhanced with 0.51 and 0.34 e beingdepleted from Mo1 and Mo2, respectively, which arelarger than that of Mo (0.07 e) in the pristine MoS2.Therefore, the number of the localized nonbonding delectrons in Mo and Mn is large resulting in a mag-netic moment of 2 lB (Table 2). Our results revealthat coupling between dopants is crucial to the elec-tronic states and magnetic polarization in monolay-ered MoS2.

Conclusion

First-principles calculation is conducted to study theeffects of (Cr ? X), (V ? B), and (Mn ? X) codopingon the electronic states and magnetic polarization inmonolayered MoS2. NM (B, N, F) doping inducesmagnetization but the materials are still semicon-ducting albeit showing a small band gap. On theother hand, TM doping gives rise to magnetism andthe magnetic moment increases with the number ofelectrons in the d orbit of the TM dopant. (Cr ? X),(V ? B), and (Mn ? X) codoping produces differenteffects depending on the total number of the outerelectrons in the Cr, V, Mn, and NM atoms. (Cr ? C)and (Cr ? O) codoping produces shallow impuritylevels near CBM and reduced band gap, whereas(V ? C), (V ? O), and (V ? B) codoping leads to aasymmetrical distribution of the spin-up and spin-down channels and magnetization. In particular,(V ? B) codoping induces the largest magneticmoment. Our results show that codoping with TMand NM atoms is an effective means to tailor theelectronic and optical properties of monolayeredMoS2.

Acknowledgements

This work was jointly supported by the NationalNatural Science Foundation of China (Grant Nos.51271139, 51471130, 51302162, 51171145), Funda-mental Research Funds for the Central Universities,and City University of Hong Kong Applied ResearchGrants (ARG) Nos. 9667104 and 9667112.

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