Tailoring Electronic and Magnetic Properties of MoS2 Nanotubes

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Tailoring Electronic and Magnetic Properties of MoS Nanotubes Nannan Li, Geunsik Lee, Yoon Hee Jeong, and Kwang S. Kim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 Mar 2015 Downloaded from http://pubs.acs.org on March 3, 2015

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The Journal of Physical Chemistry

Tailoring Electronic and Magnetic Properties of MoS2 Nanotubes

Nannan Li,† Geunsik Lee,‡* Yoon Hee Jeong,† and Kwang S. Kim‡* †

Department of Physics, Pohang University of Science and Technology, San 31, Hyojadong,

Pohang 790-784, Korea ‡

Center for Superfunctional Materials, Department of Chemistry, Ulsan National Institute of

Science and Technology (UNIST), Ulsan 689-798, Korea ⃰ Corresponding authors: E-mail: [email protected] (ksk) or [email protected] (gl)

Abstract We have studied the electronic and magnetic structures of MoS2 nanotubes by using a firstprinciples method. Various kinds of defects such as substitution and vacancy are examined for triggering spin magnetic moments towards one dimensional diluted magnetic semiconductors. Our results suggest that the presence of impurity states within the energy gap and its large contribution to the density of states at the Fermi level are the key factors in inducing a magnetic moment. In particular the nanotube curvature turns out to affect the energy level of impurity states, which can be exploited for tailoring magnetic properties. Also, 3d transition metal impurities (V, Mn, Fe and Co atoms) on a Mo site can create large magnetic moments.

Keywords Molybdenum disulfide nanotubes, magnetic property, one dimensional diluted magnetic semiconductors, first principles calculation 1

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1. INTRODUCTION Although graphene has enabled us to observe various interesting electronic, optical, and chemical properties,1-5 its intrinsic zero gap nature is often an impediment to technological applications. Several modifications have been conducted to achieve a finite band gap, such as graphene nanoribbon,6,7 dual doping,8,9 vertical electric field,10 and so on. A number of carbon nanotubes can show a finite size of band gap, however it requires post-processing to extract semiconducting nanotubes from mixed bunches of both metallic and semiconducting carbon nanotubes.11-13 Practically all of them are hardly tractable. On the other hand, in the last two decades, several layered inorganic compounds have successfully been synthesized into a single layer form, such as boron nitride (BN), molybdenum disulfide (MoS2), bismuth selenide (Bi2Se3), and so on.14-18 In particular, bulk MoS2 is known as a semiconductor with an indirect band gap, while monolayer MoS2 shows a direct gap. Monolayer MoS2 attracted much attention for probable applications such as field-effect transistors, optoelectronic devices, phototransistors, hydrodesulfurization catalysts, and solid state lubricants.19-22 Including MoS2, there is a rich variety of layered transition metal dichalcogenides, which is MX2 with M=3d, 4d, 5d elements and X= S, Se, Te. Owing to localized d electrons, it is possible for the M atom to have a permanent spin magnetic moment. Such indication was shown for M=V, Nb, Ta, where intriguing electronic properties such as enhanced Pauli magnetism, charge density wave, and superconductivity were observed. Hence, it will be of fundamental interest to study magnetic properties of MX2, and also valuable for spintronic technology application. Recently, several studies on magnetic properties of pristine and doped MoS2 nanosheets have been carried out on both experimental and theoretical levels.23-36 The magnetic measure 2


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ment23 shows that MoS2 nanosheets exhibit the ferromagnetism up to room temperature, and the magnetic ground state is attributed to the edges states. A theoretical report shows that armchair MoS2 nanoribbons have non-magnetic ground states, while zigzag MoS2 nanoribbons exhibit magnetic behavior due to the states localized at the zigzag edges.26 Recent theoretical works show that magnetic moments of MoS2 monolayer can be induced by impurities in a form of substitution or vacancy, but experimental realization is challenging.36 Meanwhile, it is well known that the dangling bonds at nanoribbon edges make the structure unstable towards rolling into curved structures, forming closed hollow structures such as fullerene-like structures and nanotubes.37 Thus, MoS2 nanotubes, instead of platelets, are better suited for practical application. For possible application as magnetic semiconductors, one needs to induce local magnetic moments and build the magnetic coupling strong enough to have ferromagnetic alignment. In this study we focus on the former part. Our theoretical study shows how to induce spin magnetic moments in a MoS2 nanotube with explanation of the origin which is not been well understood yet. In this paper, we perform calculations based on a first-principles method to predict electronic and magnetic properties and their thermodynamic stabilities of the MoS2 nanotubes with atomic substitutions or vacancy defects. It is found that atomic substitution and vacancy defects can indeed induce non-vanishing magnetic moments. In particular, the Mo substitution by 3d transition metal elements is promising, which can provide a new platform for low dimensional diluted magnetic semiconductors. 2. CALCULATION METHODS Our calculations are performed by using the Vienna Ab-initio Simulation Package (VASP).38 For modeling ion cores, we use projector augmented wave (PAW) pseudopotentials with 600 eV energy cutoff for the plane wave basis set. The exchange-correlation energy is calculated 3



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with the generalized gradient approximation (GGA) functional of the Perdew-BurkeErnzerhof (PBE) type. A k-point mesh of 1×1×7 (1×1×9) is used for sampling the Brillouin zone of armchair (zigzag) MoS2 nanotube. All of the structures are fully optimized till all forces in the supercell are less than 0.02eV/Å. The van der Waals correction is included to describe the dispersion interaction between atoms within a MoS2 nanotube, where three different functional approaches are tested: the van der Waals density functional method within optB86b functional (vdW-DF),39 the Tkatchenko-Scheffler method (DFT-TS),40 and the self-consistent screening in the Tkatchenko-Scheffler method (TS-SCS).41 It is well known that the van der Waals interaction is significant in low-dimensional nanomaterials, such as fullerenes, nanotubes, monolayer and multilayer nanostructures. Especially, in a recent report it was shown that in low-dimensional system the vdW correction method including polarization screening effects can significantly improve the conventional pairwise approximation.42 Therefore in our calculation we include and compare the above three different vdW correction methods. 3. RESULTS AND DISCUSSION Pristine MoS2 nanotubes Figure 1 shows two types of MoS2 nanotubes. Similar to carbon nanotubes they are called armchair and zigzag nanotubes for Figures 1(a) and 1(b), respectively, according to the shape along the perimeter. However, the layer comprises a S-Mo-S triple atomic layers. For both ar mchair and zigzag MoS2 nanotubes, after a full optimization, an inclusion of the vdW correcti on results in a smaller diameter than using the PBE method alone. The magnitude of reduction by the vdW-DF and DFT-TS methods are similar to each other, while the reduction by the TS-SCS method is much larger up to 0.6 Å (see Figure S1 for more details). Figures 2(a) and 2(b) show the calculated band structures of armchair and zigzag MoS2 nanotubes 4


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with the diameters of 24.6 Å and 22.0 Å, respectively. It gives an indirect/direct energy gap of ~1.0 eV for the armchair/zigzag type. These different gap characters are ascribed to the zone folding and the tube curvature or asymmetrical heights of inner and outer S atomic layers, as shown in recent theoretical studies.43,44 As shown in Figures 2(c) and 2(d), the magnitude of energy gap increases with respect to the diameter, and depends significantly on the method used to describe the dispersion interaction in the system. The TS-SCS method deviates from the others significantly, giving larger values mainly due to the screened potential. This remarkable screening effect would be due to the strong axial polarization alon g the tube axis.42 Meanwhile, the results by the vdW-DF and DFT-TS methods are similar so that the charge self-consistent effect is rather small in the MoS2 nanotube system. Our fitting of the energy gaps with a scaling function 1/R2 (R is the diameter of MoS2 nanotube) gives the values of 1.65, 1.73, 1.75, and 1.98 eV for the PBE, vdW-DF, DFT-TS, and TS-SCS methods, respectively, at the infinite limit of R. These values are comparable with the calculated energy gap (1.8eV) and the experimental gap (1.9eV)22 of the MoS2 monolayer sheet. Since the van der Waals correction is essential, we include it in the following calculations. Impure MoS2 nanotubes For studying the effect of substitution and vacancy defects on the electronic and magnetic structures of a MoS2 nanotube, we choose an armchair nanotube with a diameter as large as 24.6 Å to avoid serious deformation on the tube due to the defects, where the same trend is expected for other nanotubes with different chirality and diameter. In Figure 3, we show the electronic density of states (DOS) for the chosen tube. The total DOS in Figure 3(a) shows that the pristine system is an insulator with an energy gap of about 1.0 eV. From the projected DOS shown in Figure 3(b), one can see strong hybridizations between Mo-d and S-p orbitals 5



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roughly below -1.5 eV and above 3.5 eV which correspond to the covalent type bonding and anti-bonding states, respectively. Between them, one can see Mo-d derived states split by the ligand field of the slightly distorted trigonal prismatic coordination (D3h), where the three energy levels out of five degenerate d states are expected. The lowest one, i.e. the top valence band, has mainly the dz2 character, while the rest four orbitals contribute to those states near the bottom of conduction bands. Since two electrons of original four d electrons in a Mo atom remain non-bonding, the lowest dz2 band is fully occupied with substantial energy gap above it. Interestingly, at the higher energy edge of the dz2 band there exist valence band tail states. Figure 3(c) shows that such tail states are not present in the MoS2 monolayer system, so the nanotube curvature induces another band at the valence top, similar to what has been reported for a monolayer under in-plane strain.44,45 When defects are introduced in the pristine nanotube, we consider a supercell including three unit cells along the tube axis to minimize the interaction between impurities. It means the substitution of one S or Mo atom among 216 atoms, which amounts to 0.46 at.%. The atomic structures are depicted in Figure 4. S substitution For substitution of an S atom we consider four elements around S in the periodic table, which are Si (metalloids group), P (pnictogen group), Cl and Br (halogen group). An atom on the outer S atomic layer is chosen to be substituted. After full geometry optimization, there is no dramatic distortion on each impure nanotube, as shown in Figure 4(a). The detail of the structures shows that the Si and P substitutions slightly shrink the bonds around the impurities, though the Cl and Br atoms enlarge the bonds, as listed in Table 1. For the relaxed geometry of each case, the spin-polarized calculation have been performed to check the formation of spin magnetic moment. As shown in Table 1, it turns out that the Si or P doped 6


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MoS2 nanotube has the magnetic ground state with ~0.8 µB, while the Cl or Br does not. Also we list the total energy difference between non-spin-polarized and spin-polarized calculations, where the total energy is significantly lowered upon having a finite magnetic moment. In Figure 5, one can see the effect of P or Cl substitution on the electronic DOS, where Si and Br have almost the same features as P and Cl, respectively. The overall feature is that the hole carriers are induced by P (or Si) atom as shown in Figure 5(a), whereas the electron carriers are induced by Cl (or Br) as shown in Figure 5(c). The orbital character at the Fermi level will be responsible for the magnetic behavior of our results, i.e. magnetic/non-magnetic ground state for the hole/electron doping. In the case of hole doping (Figure 5(b)), the states near the Fermi level contain a significant hybridization of the P-p orbital together with Mo-d and S-p orbitals, which indicates P-related impurity states about 0.1 eV above the top of valence band. On the other hand, the electron doping, as shown in Fig. 5(d), shows that the Cl-related impurity states appear about 0.25 eV above the conduction band bottom. From our results, the magnetic moment or exchange splitting is not vanishing when the impurity level appears within the energy gap like the Si and P cases. It is further supported by a recent theoretical report where the Cl doped MoS2 monolayer having a finite magnetic moment retains the impurity level within the gap in contrast to our nanotube case.24 It is notable that the nanotube and monolayer of MoS2 exhibit different impurity levels or magnetic behavior under electron doping. Our result of MoS2 nanotube suggests that the hole doping via substitution of an S atom is more likely to have a magnetic ground state than the electron doping. Mo substitution In the case of the atomic substitution on a Mo site, we consider the transition metals V, Nb, 7



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Ta, Mn, Ru, Re, Cr, Fe, and Co which are known to form dichalcogenides. Three of them (V, Nb, Ta) have one less electron than Mo, and another three (Mn, Re, Ru) have one or two electrons more. The others (Cr, Fe, Co) are known to form (anti)ferromagnetic materials. The total DOS’s are shown in Figure 6 for the V, Nb, Ta, and Mn, Re, Ru doped cases. The magnetic moments obtained by spin-polarized calculations are listed in Table 1. Although the substitution effect of a S atom on the magnetic property does depend on the doping type, that of a Mo atom does not. All impurities induce the gap states near the valence top or the conduction bottom, but most of them are not magnetic even for the deep gap states by Ru. Instead, only 3d elements exhibit non-zero spin moments. It is because strongly localized 3d orbitals are more likely to induce spin moments due to strong Hund coupling which is competing with ligand field energy splitting. In the case of the other three 3d elements, the calculated spin moments for the Cr, Fe, Co substituted MoS2 nanotube are 0, 2, 1 µB, respectively (see Table 1). These values are underestimated, because it is known that the GGA method is inaccurate in describing the Coulomb correlation effect for 3d elements. Inclusion of such effects via the GGA+U method with U = 5 eV46 gives enhanced moments for V and Co, as the results for the 3d elements are listed in Table 1. In the case of Cr, we have seen non-zero moments obtainable at higher U values. Figure 7 shows the total DOS for the Co substitution case with and without including U. One can see that the gap states caused by the impurity is split more in spin up and down energies by including U, which indicates the major role of the Coulomb effect. Vacancy defects Furthermore we consider the single atom (S or Mo), double atom (2S) and triple atom (MoS2) vacancy defects, where the structures are displayed in Figure 8. Due to the missing atoms, the Mo-S bonds around the defect sites are all shrunk compared to the perfect tube. 8


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Although the vacancy defects cause strains near their sites, there is no obvious deformation of the nanotubes. As listed in Table 1, a single atom vacancy defect on a Mo site induces a spin moment of 0.6µB. However, either mono-vacancy or di-vacancy of S does not cause a spin moment. Meanwhile, a triple MoS2 defect induces a moment of 2 µB. As one can see the DOS in Figure 9, the spin splitting is achieved in the case of missing Mo or MoS2 unit, while all the four cases show the vacancy induced states at a certain energy level within the gap. This behavior can be understood by counting the number of electrons involved in the dangling bonds. The formal valences of Mo and S are +4 and -2, respectively. Since a Mo has six neighbors of S, it contributes 2/3 electrons for each Mo-S covalent bond. When one S is missing, the dangling bonds of three nearest neighbor Mo atoms result in 3 x 2/3 = 2 lone pair electrons. When two S’s are missing, there are four dangling bond electrons. The even number of electrons means an equal occupation of spin up and down states, assuming negligible contribution of Hund coupling. Thus it gives a zero spin moment. In the case of one missing Mo atom, similar argument is applicable, i.e. 3x2/3=2 electrons deficiency for each of outer and inner S atomic layers. Although it is an even number of electrons, the asymmetry in the outer and inner bond lengths would cause a finite magnetic moment, which will be the same for the missing MoS2 unit. We further confirmed that the missing Mo in monolayer does not cause a magnetic moment.36 The p-type doping substitution of S atom (Si-MoS2/P-MoS2) can trigger spin splitting between the spin up and down channels, leading to magnetic property of the MoS2 nanotube. Using 3d elements V, Mn, Fe or Co to replace the Mo atom on the tube is another possible strategy for tailoring the magnetic properties of MoS2 nanotubes. Furthermore, the single Mo atom vacancy defect and the triple MoS2 atom vacancy defect in the MoS2 nanotube can also determine the magnetic ground state of the nanotube.

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To study the magnetic coupling between the impurities in a MoS2 nanotube, we place two impurities in various configurations with different distance between them and direction, and calculate the exchange energy between the two impurities by comparing the total energies between ferromagnetic (FM) and antiferromagnetic (AFM) states for each case (details of the structures and results are given in Figure S2 and Table S1). The distance between magnetic impurities varies 3.2 Å to 10.9 Å. According to our calculated results, the exchange energy between two Fe impurities can be as large as 122.8 meV and 210.9 meV in the armchair and zigzag MoS2 nanotubes, respectively, which shows a high possibility to produce ferromagnetism on impure MoS2 nanotubes. But, for other cases like S substitution or vacancy defects, it is negligible. Formation energies We further calculate the formation energy of each substituted nanotube to assess the synthesis possibility. The formation energy of an atomic substitution in the MoS2 nanotube is:

E form = Esub + N µMo / S − E perf − N µimp where Esub is the total energy of the substituted MoS2 nanotube, E perf is the total energy of the perfect pristine MoS2 nanotube, µMo/ S is the chemical potential of a Mo or S atom, µimp is the chemical potential of the impurity, and N is the number of substituted atoms. On the other hand, the formation energy of the vacancy defect in the tube is:

E form = Evac + N µMo / S / MoS2 − E perf where Evac is the total energy of the MoS2 nanotube with the vacancy defect, N is the number of defect atoms, µMo / S / MoS2 is the chemical potential of a Mo atom, S atom, or MoS2 molecule. We calculate the formation energy for two limiting cases of the chemical environment, enriched by either Mo or S atoms. For the Mo (S) rich environment, the chemical potential of 10


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Mo (S) atom is calculated from the corresponding unary bulk system, while the chemical potential of S (Mo) atom is obtained from the thermodynamic equilibrium condition

µMoS = µMo + 2µS .24 Here the chemical potential of a MoS2 molecule µMoS is obtained from 2

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the perfect MoS2 nanotube. The calculated formation energy for each defect is listed in Table 2. It shows that the S atom substitution and vacancy are easier to be formed under Mo rich environment, whereas the Mo atomic substitution and vacancy are easier under S rich condition. Also, compared to the formation of vacancy defect, the atomic substitution is more energetically favorable. We notice that the impurity formation in nanotubes is more favorable than that in a monolayer from our calculation, where the formation energy is higher for the monolayer case by 0.3~0.6 eV for the atomic substitution or by 1.2 eV for the Mo vacancy. As well known, the production of macroscopic quantities of MoS2 nanotubes is normally carried out at high temperatures (800-950℃ by Tenne et al.47 and ~1300℃ by Nath et al.48). Therefore, the high temperature synthesis can provide a significant energy for the atomic substitution mentioned in this work, and even trigger vacancy defects in MoS2 nanotubes. Actually the fullerene-like NbxMo1-xS2 nanoparticles and Mo(W)1-xRexS2 nanoparticles have a lready been observed in experiment.49-51 This means that the defect substituting a Mo atom could give a spin moment. Furthermore, recent experimental observations support the formation of vacancy defects on MoS2 monolayer in either extrinsic or intrinsic form.33,52,53 We have also examined the possibility of clustering by comparing the formation energy of the first impurity and the second impurity near the first one. For the P substitution case, the formation energy of the second impurity is 0.13 eV lower than the first one, which indicates that clustering of impurity atoms is possible. 4. CONCLUSIONS In this work, we have presented a systematic study of pristine and defected MoS2 nanotubes 11



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as possible applications towards one dimensional diluted magnetic semiconductor. Our calculations show that finite spin magnetic moments can be induced by introducing various kinds of defects such as substitution and vacancy into the semiconducting pristine nanotubes. Interestingly we have seen that the hole doping via a S-site substitution triggers a magnetic state, while the electron doping does not. Such peculiar behavior is ascribed to the presence of impurity gap states whose level depends on impurity species and interestingly the curvature as well.54 Meanwhile, 3d transition metal impurities (V, Mn, Fe and Co atoms) on a Mo site can create larger magnetic moments for the nanotubes. The finite magnetic moments are also observed for the MoS2 nanotubes with a single Mo atom and triple MoS2 atoms vacated. According to the formation energy of each substituted or defected MoS2 nanotube, the magnetic MoS2 nanotube would be realistic during the synthetic procedure. Thus, the half metallic property could be realized in a MoS2 nanotube by using aforementioned methods.

ACKNOWLEDGMENTS This work was supported by the NRF (Basic Science Research Program: 2011-0010186 and National Honor Scientist Program: 2010-0020414), KISTI (KSC-2014-C3-019, KSC-2014C3-020), and the National Research Foundation via SRC at POSTECH (2011-0030786).

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Table 1. Magnetic moment (Μtotal) and the total energy difference between non-spinpolarized and spin-polarized calculations ( ∆E = Espin − Enon − spin ) for the atomic substitution defect of a S/Mo atom and the vacancy defect at a S/Mo site in the MoS2 nanotube, and the optimized bond lengths (D) around each defect. For each Mo substitution by a 3d element, a magnetic moment by the GGA+U method is listed in the bracket.

System

ΔE (meV)

Μtotal (µB)

D (Å)

atomic substitution of a S atom Si

0.78

-77.40

2.42, 2.47

P

0.76

-78.20

2.42, 2.47

Cl

0

0

2.51, 2.66

Br

0

0

2.64, 2.77

atomic substitution of a Mo atom Nb

0

0

2.46, 2.53

Ta

0

0

2.45, 2.50

Ru

0

0

2.43, 2.47

Re

0

0

2.40, 2.45

-73.70

2.37, 2.42

V

0.16 (1)

Cr

0 (0)

0.00

2.35, 2.40

Mn

1 (1)

-161.70

2.35, 2.36

Fe

2 (2)

-210.80

2.33, 2.39

Co

1 (3)

-155.20

2.34, 2.34

vacancy defect S

0

0

2.34, 2.35

2S

0

0

2.42, 2.44

Mo

0.60

-75.70

2.39, 2.40

MoS2

2

-148.70

2.35, 2.48

Pristine Nanotube

0

0

2.43, 2.49

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Page 20 of 32

Table 2. Formation energies (Eform) for the atomic substitution defect of a S/Mo atom and the vacancy defect at a S/Mo site in the MoS2 nanotube under Mo/S atom rich experimental environment. Eform(eV)

System Mo atom rich

S atom rich

atomic substitution of a S atom P

0.39

1.56

Cl

-0.14

1.03

Si

-0.94

0.23

Br

0.17

1.34

atomic substitution of a Mo atom Re

1.80

-0.54

Ru

2.72

0.38

Nb

-0.61

-2.95

Fe

2.69

0.35

Co

3.56

1.22

Ta

-0.54

-2.87

V

-0.04

-2.37

Mn

1.60

-0.74

Cr

0.62

-1.72

vacancy defect S

1.99

3.16

2S

2.84

5.17

Mo

6.13

3.79

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Figure Captions

Figure 1. Top and side views of (a) armchair and (b) zigzag MoS2 nanotubes. The shortest periodicity along the tube axis is shown in each figure with the dashed line.

Figure 2. Band structures for (a) armchair and (b) zigzag MoS2 nanotubes. The arrow indicates an indirect or direct energy gap, respectively. (c) and (d) are the calculated energy band gaps for armchair and zigzag MoS2 nanotubes by varying the diameter with the PBE functional and different vdW correction methods.

Figure 3. (a) Total and (b) projected density of states for a pristine armchair MoS2 nanotube of a diameter 24.6 Å; (c) Total density of states for the MoS2 monolayer. The Fermi energy is set to 0 eV.

Figure 4. Side views of an armchair nanotube with a diameter 24.6 Å, where atomic substitution defects are introduced. Cyan balls indicate Mo atoms, yellow balls are S atoms, blue and red balls are the impurity atoms. (a) depicts a S atom in the three-unit supercell substituted by one of Si, P, Cl, Br, as discussed in the text, while (b) depicts a Mo atom substituted by one of V, Nb, Ta, Mn, Ru, Re, Cr, Fe, and Co. The supercell is marked with dashed line.

Figure 5. (a) Total and (b) projected density of states for the single S atom substitution by P (hole doping) in the MoS2 nanotube. The same results for the substitution by Cl (electron 21



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doping) are given in (c) and (d).

Figure 6. Total density of states for the cases where a single Mo atom is substituted by an atom of (a) V , (b) Nb, (c) Ta, (d) Mn, (e) Re, and (f) Ru in a MoS2 nanotube.

Figure 7. Total density of states for the cases where a single Mo atom is substituted by a Co atom in the MoS2 nanotube, using (a) GGA and (b) GGA+U methods.

Figure 8. Atomic structures for vacancy defects in the site of (a) a single S atom, (b) a single Mo atom, (c) double S atoms, and (d) triple MoS2 atoms in a MoS2 nanotube. Note substantial reconstruction in cases of (c) and (d).

Figure 9. Total density of states for vacancy defects in a MoS2 nanotube (refer to Figure 8 for the atomic structure): (a) a single Mo atom, (b) triple MoS2 atoms, (c) a single S atom, and (d) double S atoms defects.

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(a)

(b)

Figure 1

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(a)

(c)

(b)

(d)

Figure 2

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(a)

(b)

(c)

Figure 3 Figure 3

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(a)

(b)

Figure 4

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(a)

(c)

(b)

(d)

Figure 5

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(a)

(d)

(b)

(e)

(c)

(f)

Figure 6

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(a)

(b)

Figure 7

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(a)

(c)

(b)

(d)

Figure 8

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(a)

(c)

(b)

(d)

Figure 9

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For Table of Contents Only

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Tailoring Electronic and Magnetic Properties of MoS2 Nanotubes

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