Structural and Electronic Properties of Superlattice Composed of

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Structural and Electronic Properties of Superlattice Composed of Graphene and Monolayer MoS 2

X. D. Li, S. Yu, Shunqing Wu, Yu-Hua Wen, S. Zhou, and Zi-Zhong Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp404080z • Publication Date (Web): 02 Jul 2013 Downloaded from http://pubs.acs.org on July 4, 2013

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The Journal of Physical Chemistry

Structural and Electronic Properties of Superlattice Composed of Graphene and Monolayer MoS2 X. D. Li1, S. Yu1, S. Q. Wu1,*, Y. H. Wen1, S. Zhou2, Z. Z. Zhu1,2,3* 1

Department of Physics and Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China 2 State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, China 3 Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen 361005, China

Abstract Hybrid systems consisting of graphene and various two-dimensional materials would provide more opportunities for achieving desired electronic and optoelectronic properties. Here, we focus on a superlattice composed of graphene and monolayer MoS2. The geometric and electronic structures of the superlattice have been studied by using density functional theory. The possible stacking models, which are classified into four types, are considered. Our results revealed that all the models of graphene/MoS2 superlattices exhibit metallic electronic properties. Small band gaps are opened up at the K-point of the Brillouin zone for all the four structural models. Furthermore, a small amount of charge transfer from the graphene layer to the intermediate region of C-S layers is found. The band structure, the charge transfer together with the buckling distortion of the graphene layer in the superlattice indicates that the interaction between the stacking sheets in the superlattice is more than just the van der Waals interaction. Keywords: Band gap, Charge transfer, van der Waals, First-principles calculations

* Corresponding authors. Tel. +86-592-2182248 Email address: [email protected] (Z. Z. Zhu); [email protected] (S. Q. Wu) 1

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1. Introduction Graphene, a typical example of two-dimensional (2D) atomic-layer-based materials, is a zero-gap semiconductor with a point-like Fermi surface and a linear dispersion at the Fermi level.1-3 It has attracted enormous attention in the past several years due to its fascinating properties, such as massless Dirac fermions,4 unique transport properties and intriguing quantum Hall effect,2 which makes graphene a promising material in digital electronics, pseudo-spintronics5 and infrared nanophotonics.6 Since the presence of a band gap is a crucial property in electronic and/or optoelectronic devices, it is significant to introduce a band gap in graphene. This problem could be solved by doping, tailoring or controlling the dimensionality of graphene,7-9 which, however, usually result in crystallographic damage to the materials and unexpected loss in the excellent electrical properties.10-14 Therefore, it is important to find a solution that not only brings a tunable band gap but also reserves the integrity of graphene. Recent studies have shown that hybrid systems consisting of graphene and various two-dimensional materials would provide more opportunities for achieving desired electronic and/or optoelectronic properties.10,11,15 Therefore, we introduce molybdenum disulfide (MoS2) into our system of study due to its novel band structures and focus on the superlattice composed of graphene and monolayer MoS2. Bulk MoS2, a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. As the number of MoS2 layers decreases, the fundamental indirect band gap increases.16,17 When thickness of the MoS2 slab is reduced to form a 2D monolayer, it changes into a direct band gap semiconductor. The values of the natural band gaps turn from 1.2 eV (indirect) in the MoS2 2


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bulk to 1.9 eV (direct) in the MoS2 monolayer.18-21 To our best knowledge, no theoretical work has been done for the physical properties of monolayer MoS2/graphene superlattice, including its electronic, magnetic and structural properties. The induced band gap caused by symmetry breaking has been found in epitaxial graphene layers on SiC.10 The mechanically tunable

band

gap

and

quasi-particle-effective-mass

has

been

realized

in

graphene/hexagonal-BN hetero-bilayer by application of in-plane homogeneous biaxial strain.15 Moreover, interesting experimental studies have also been performed in some MoS2/graphene hybrid systems recently.22-25 The three-dimensional (3D) hybrid structure, made with layered MoS2 supported on the graphene surface, not only enhances the stability of the MoS2/graphene composites due to superstrength of graphene but also form an interconnected conducting network for the less-conducting MoS2.22 MoS2/graphene nanosheet and graphene-like MoS2/amorphous carbon composites have been synthesized by Chang et al., 23-24 which increase the distance of the original graphene nanosheets and MoS2 layers. Thus, the new system can provide larger space for Li ion intercalation, which also reduces the barriers to Li ion mobility. The electrochemical tests on the MoS2/graphene composites demonstrated that the capacities of the composites were higher than any of the components and the cycle stabilities were also excellent (> 100 cycles), which suggested that they were promising anode materials for lithium ion batteries.22-24 The studies on the novel nanocomposites comprised of single-layer MoS2, graphene and amorphous carbon showed not only the high conductivity of the composite but also the large enhancement of the electron rapid transfer.25 Thus, it seems that 2D materials could complement each other and that had been the case so far. 3



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Here, we report a density functional theory (DFT) study of the structural and electronic properties of a superlattice made with alternate stacking of graphene and monolayer MoS2. Four possible candidate stacking models of the superlattice are considered. The geometries, binding energies, relative stabilities, total and partial density of states (TDOS and PDOS) and band structures of these different structural models are discussed. Our results show that all the four superlattice models are insensitive to the spin-polarized effect and exhibit metallic electronic properties. In addition, small band gaps open up at the K-point of the Brillouin zone (Dirac point) for all the four structural models, which are due to the interactions between the stacking graphene layers. Small amounts of charge transfer between atomic layers are found, indicating also a weak ionic interaction between the graphene and the MoS2 layers, suggesting more than just the van der Waals ones between the stacking sheets in the superlattices.

2. Methods All calculations are carried out by using the projector augmented wave (PAW) method26 within the density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP).27,28 The exchange-correlation functional is treated with the local density approximation.29,30 For a better account of the weak interlayer attractions in the present layered superlattice, we have also performed PBE-D2 calculations,31 in which the van der Waals (vdW) interaction is taken into account by adding a semi-empirical dispersion potential to the conventional Kohn-Sham DFT energy. Our results show that the band structures obtained by LDA and PBE-D2 calculations are very close to each other. In the paper, we 4


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show the LDA results only. Wave functions are expanded in plane waves up to a kinetic energy cutoff of 600 eV. Brillouin-zone integrations are approximated by using the special k-point sampling of Monkhorst-Pack scheme32 with a Γ-centered 5×5×3 grid. The cell parameters and the atomic coordinates of the superlattice models are fully relaxed until the force on each atom is less than 0.01 eV/Å. Although the spin-polarized calculations are performed, the results indicate that all the superlattice models do not exhibit magnetism at their equilibrium lattice constants. The calculated lattice constant of a free-standing graphene is 2.45 Å, which agrees well with the experimental value of 2.46 Å33 (where the lattice constant: a graphene = 3d c − c , with dc-c is the carbon-carbon bond length). The optimized lattice constant of a single MoS2 monolayer is 3.13 Å, which is consistent with the reported value of 3.13 Å18 (where the lattice constant of a MoS2 monolayer equals to the Mo-Mo bond length). The calculated x-y plane lattice constant of bulk MoS2 (ABAB stacking) is 3.12 Å, which is very close to the counterpart of a MoS2 monolayer (3.13 Å), in consistent with the 2D nature of the bulk MoS2. Although the lattice constants of graphene and monolayer MoS2 are quite different, both of them do share the same primitive cell of hexagonal structure. To minimize the lattice mismatch between the stacking sheets, for the superlattice structures, we have employed supercells consisting of 4×4 unit cells of graphene and 3×3 unit cells of MoS2 monolayer in the x-y plane. Thus, 4a gra = 9.80 Å and 3a MoS 2 = 9.39 Å, which leads to a small lattice mismatch (less than 4.5%) in the supercell initially. First, the lattice of graphene ( 4a gra ) was set to match to that ( 3a MoS 2 ) of monolayer MoS2 in the supercell. The supercells are then fully relaxed for both the lattice constants and the atomic geometry. The mismatch will finally 5



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disappear, leading to the commensurate systems. The relaxed lattice constants of all the four superlattices are around 9.672 Å. Comparing to the corresponding isolated sheet, the graphene layers in the superlattices are compressed by 1.3% (from 2.45 Å to 2.418 Å), while the MoS2 layers in the supercells are expanded by 3.0% (from 3.13 Å to 3.224 Å) (see Table 1).

Table 1. Geometries and binding energies of the superlattices with four stacking models, including the interlayer distance of graphene and adjacent molybdenum layer (d1), the interlayer distance between two adjacent graphene or two molybdenum layers (d2) and amplitude of buckling in the graphene layers (∆)

2.765 2.766 2.838

dC–C (Å) 1.396 1.396 1.396

dMo–S (Å) 2.406 2.405 2.404

2.844

1.396

2.403

Stacking

a (Å)

Eb (eV/supercell)

AC1AC1 AC1AC2 AC1BC1

9.673 9.673 9.671

AC1BC2

9.671

d1 (Å)

d2 (Å)

∆(Å)

4.845 4.823, 4.861 4.829, 4.848 4.809, 4.858, 4.828, 4.819

9.707 9.685 9.675

0.019 0.012 0.168

9.667

0.180

3. Results and Discussions In the present study, we introduce a superlattice by hybridizing monolayer MoS2 with graphene. This hybrid maintains the integrity of graphene. The superlattices consist of alternate stacking of graphene and monolayer MoS2 layers, with two graphene sheets and two single MoS2 layers per supercell. Here, we use C1, C2 and A, B to represent the two graphene layers and two single layers of MoS2 in the supercell, respectively (see Fig. 1(a)). For graphene in the supercell, two types of stacking models are considered: (1) Hexagonal stacking (denoted by C1C1-type), where all the carbon atoms in C1-layer lie above the other C1-layer; (2) Bernal stacking (denoted by C1C2-type), where C2-layer is constructed from a rotation of C1-layer by 60º (the origin of rotation is on a C atom of layer-C1). For the MoS2 6


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layers in the supercell, we have also considered two stacking models: the AA and AB stacking. For the AA stacking (denoted by AA-type), all sulfur atoms of one MoS2 layer lie above the sulfur atoms of the other layer. For the AB stacking (denoted by AB-type), however, all the sulfur atoms in one MoS2 layer lie above the molybdenum atoms of the other MoS2 layer. There are 16 configurations in total. Through structural analysis and classification, it turns out that only four kinds of stacking models are independent, i.e. AC1AC1, AC1AC2, AC1BC1 and AC1BC2, as shown in Fig. 1(b) and 1(c).

Fig. 1 (Color online) (a) Top view of the two single MoS2 monolayer and the two graphene sheets used in the supercell; (b) Side view of the four different stacking models; and (c) Four different stacking configurations. C, Mo and S atoms are represented by grey, purple and yellow balls, respectively. 7



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The optimized structural parameters of the superlattices studied are listed in Table 1. For the four superlattices considered, the lattice constants (in the x-y plane) are almost the same (9.672 Å), therefore, the bond lengths of C-C in the graphene layer and those of Mo-S in the MoS2 layer are also around the same values, i.e., 1.396 Å and 2.405 Å, respectively. Comparing to the calculated C–C bond length in an isolated graphene (1.41 Å) and Mo–S bond length in a MoS2 monolayer (2.38 Å), it is clear that, in the superlattices, C-C bond lengths are compressed by 1.3%, while Mo-S bond lengths are expanded by 1.0%. Although atomic bond lengths within the stacking planes are almost the same for all the four superlattices, however, the interlayer distances exhibit relatively larger deviations (but still very close to each other). The interlayer distances between graphene and adjacent molybdenum layers (denoted by d1) are found to be all around 4.80 Å. There are four kinds of C-Mo distances in the AC1BC2-stacking model, i.e., C1-A, C1-B, C2-A, and C2-B, therefore, four values are presented in Table 1. The shortest d1 of all the four models is in the AC1BC2-stacking model (C2-A), indicating that the C2-A layers have the strongest interaction. The distance between C-Mo layers, 4.80 Å, is comparable to the experimental value for the MoS2/amorphous carbon composites, 5.20 Å.23 The interlayer distances of Mo–Mo or C–C layers (denoted by d2) are also presented in Table 1. It is clear that AC1BC2-stacking model has the shortest d2, showing again the strongest bonding among the four models. Since the AC1AC1-stacking has the largest d2, it is believed that such a stacking should not be favored in energy. The conclusions reached above are in consistent with the fact that for the two-layer stacked graphene, the C1C2 stacking is more favorable than the C1C1 one.34 Also, the crystal structure of 2H-MoS2 is AB stacking rather than AA 8


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stacking.35,36 Furthermore, the graphene layers are found to be buckling distorted, and the amplitude of the buckling ∆ are also listed in Table 1. It shows that the ∆ becomes significant only for the superlattices with AC1BC1 and AC1BC2 stacking. The shortest distances between the graphene and MoS2 layers, i.e., the C-S atomic distances, in all the four superlattices are around 3.36 Å, which is much larger than 1.81 Å, the sum of the atomic radii of carbon and sulfur when in covalent bonds (0.77 Å for carbon and 1.04 Å for sulfur, respectively). Such a large distance of C-S atoms can indicate that van der Waals interactions could be the primary interactions in the superlattices. To discuss the relative stabilities of the superlattices, the binding energy between the stacking sheets in the superlattice is defined, such as Eb = Esup ercell − 2( E MoS2 + EGra ) , where

Esup ercell is the total energy per supercell, and E MoS2 and E Gra are the total energies of an isolated MoS2 monolayer and an isolated graphene sheet, respectively. E MoS2 is calculated by using a 3×3 unit cell of the MoS2 monolayer and E Gra is calculated by using a 4×4 unit cell of the graphene (i.e., the size of the unit cell are the same as the supercell of the superlattice). The factor 2 in the equation corresponds to two layers of MoS2 and two graphene sheets in the supercell. The present study reveals that the binding energy between the stacking layers of superlattice are very small, e.g., 0.044 eV/C-atom or 0.158 eV/MoS2 for the AC1BC2-stacking model (see Table 1). The binding energy between atomic layers in the bulk MoS2 is 0.114 eV/MoS2, which is a bit smaller than that in the superlattice. The calculated binding energy differences between the four superlattice models are also small (see Table 1), indicating again weak interactions between the atomic layers among all the superlattice models. The binding energies with AB–type packing are shown to be larger than those of the 9



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AA–type packing. The relationship between the binding energies of the four superlattices are EAC1BC2 > EAC1BC1 > EAC1AC2 ≈ EAC1AC1, where the binding energies of AC1AC1- and AC1AC2-stacking model are very close. The AC1BC2-stacking model has the largest binding energy, showing again the most stable and strongest bonding in the AC1BC2-model. The binding energy shows consistent information as the lattice parameter does, i.e., the shortest d2 of the AC1BC2-stacking has the largest binding energy. The very small differences in the binding energies as well as the very small geometric differences (i.e., differences in bond lengths and lattice parameters, as discussed above) between the four superlattices indicate that the interactions between the stacking layers of the superlattices should be weak. From the electronic structures of isolated graphene and monolayer MoS2, such a weak interaction, mainly van der Waals type, can be expected. Figure 2 illustrates the electronic band structures of the four superlattice models considered. The superlattices with different stacking are all shown to be metallic. The Fermi levels are shifted down to below the Dirac point, that is, the fully occupied band (around the Dirac point) in the isolated graphene is now partially occupied in the superlattice. For a detailed analysis of the superlattice band structure, the contribution of graphene and monolayer MoS2 to the band structure of superlattice (AC1BC2 stacking) are shown in Fig. 3(b). The band structures shown in Fig. 2 and Fig. 3(b) suggest clearly the charge transfer from the graphene layer to the MoS2 sheet. Moreover, a small band gap (δ) slightly above the Fermi level opened up at the K point of the Brillouin zone, and the linear bands around the Dirac point of graphene appear to be preserved. The gaps opened for the AC1AC1 and AC1BC1 configurations are 30 and 31 meV, respectively, however, for the AC1AC2 and 10


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AC1BC2 superlattices, the gaps are close to zero. There are two kinds of gaps (δ) in the four models of superlattices. The difference between their band structures can be told from their corresponding Dirac cones (see Fig. 2). The bands around the gap for the Hexagonal C1C1-stacking superlattices are similar to the interpenetrated ones in the bilayer graphene, while the bands around the gap for the Bernal C1C2-stacking ones are similar to the nested ones in the bilayer graphene.37,38 Since the electronic states around Fermi level at K-point are all contributed from graphene layers (see Fig. 3(b)), the gaps opened for the superlattices are then due to the interactions of graphene layers only. The stacking pattern of graphene layers (C1C1 and C1C2-) is the key factor to the size of δ, while the stacking of MoS2 layers (AAor AB-) show little influence on the band gap δ. The formation and the size of the small band gaps opened in the superlattices can be explained based on the well-known theory of bilayer graphene37. It is well known that LDA underestimates significantly the energy gap due to the intrinsic shortcoming of DFT. Hence, the values that we obtained just put a lower bound on the band gap opening. The band structure of graphene is sensitive to the lattice symmetry. If the layered hexagonal structure is composed of nonequivalent atoms, such as in BN, the in-plane symmetry is broken, resulting in the formation of a large gap between π and π* states. The symmetry can also be broken with respect to the c-axis by alternate stacking of graphene and monolayer MoS2 sheets (in this work). Such a symmetry breaking should also lead to band gap opening at the Dirac point. However, due to the very weak interaction between the graphene and MoS2 sheets, the gaps opened are all very small (see Fig. 2). Such small gaps may have no important significance for real applications, and the importance should be only 11



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in the theoretical respect. Our results suggest that strong interactions between graphene and extraneous materials (in superlattice) are necessary in order to open a large band gap.

Fig. 2 (Color online) Band structures of the four superlattices with different stacking. Detailed band structures in the vicinity of band gap opening are inserted. Red dashed line represents the Fermi level.

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Fig. 3 (Color online) (a) Band structures of isolated graphene and monolayer MoS2, respectively. (b) Contribution from graphene and monolayer MoS2 to the band structure of superlattice is shown with circles, in the left and right panels of the plot, respectively. (c) Density of states for the superlattice with AC1BC2-stacking. The Fermi level is set to be at 0 eV.

We now have a further discussion on the electronic structures of AC1BC2-stacking superlattice, since the AC1BC2 is the most stable stacking pattern. The calculated band structures and the total density of states (TDOS) are presented in Fig. 3(b)-(c). The contributions of graphene and MoS2 monolayer to the band structure of AC1BC2 superlattice are also shown with circles in Fig. 3(b), where the size of circles are proportional to the contributions. The bands near the Dirac cone are totally contributed from graphene, while the dispersive bands along Γ–A near the Fermi level are contributed from the MoS2 layers. For comparison, the band structures of pristine graphene and MoS2 monolayer are also given in 13



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Fig. 3(a). Isolated graphene is a zero-gap semimetal, while monolayer MoS2 is a direct band gap semiconductor. What discussed above for the AC1BC2-stacking superlattice have shown that: (1) the superlattice is metallic, (2) there is charge transfer between atomic sheets, (3) a small band gap opened up at the K point, and (4) the linear bands around the Dirac point are preserved. In addition, Fig. 3(b) shows that beside the bands along the Γ-A (which is along the z-direction) cross the Fermi level, the bands in the directions of M-K and K-Γ (which are directions in the x-y plane) also cross the Fermi level. This feature suggests that electrons can also be feebly conductive (DOS at Fermi surface is very small) on the graphene and MoS2 planes besides along the direction perpendicular to the stacking sheets. To further explore the bonding nature and the charge transfer in the AC1BC2-stacking superlattice, the contour plots of the deformation charge density ( ∆ρ1 ) of planes passing through graphene, S and Mo layers in the superlattice are shown in Fig. 4(a)-(c). The r r r r deformation charge density ∆ρ1 is defined as ∆ρ1 (r ) = ρ (r ) − ∑ ρ atom (r − Rµ ) , where µ

r ρ (r ) represents the total charge density of the superlattice and

∑µ ρ

atom

r r ( r − Rµ ) are the

superposition of atomic charge densities. The charge density differences shown in Fig. 4(a)-(c) exhibited that the formation of the graphene/MoS2 superlattice did not distort the charge densities of graphene, S and Mo layers by a significant amount. The charge density differences ( ∆ρ 2 ) on a plane perpendicular to the graphene or MoS2 layers and passing through Mo and two S atoms are shown in Fig. 4(d). The charge density difference ∆ρ 2 is

r r r defined as ∆ρ 2 ( r ) = ρ ( r ) − ρ slab ( graphene) − ρ slab ( MoS 2 ) , where ρ (r ) , ρ slab (graphene) ,

ρ slab ( MoS 2 ) are charge densities of the superlattice, the graphene and the MoS2 slabs, respectively. All the charge densities here are calculated by using the same supercell for the

14


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superlattice. Such a ∆ρ 2 can clearly demonstrate the charge transfer between the stacking layers in the superlattices. Charge transfer from the graphene sheet to the intermediate region between the graphene and MoS2 layers is distinctly recognizable in Fig. 4(d). Within the MoS2 layer, however, there is little change in the charge distributions. Fig. 4(e) is the deformation charge density ( ∆ρ1 ) on a plane parallel to and 0.5 Å (which is expected to see the conjugated big π bond clearly) above the graphene layer of the AC1BC2 superlattice; please refer to the illustration in the Fig. 4(f) for the plane plotted. The picture of conjugated big π bond is preserved in Fig. 4(e), which is very close to the one in the isolated graphene (not presented). The small amount of charge transfer should result in weak ionic interactions between the graphene and the MoS2 sheets. Such a charge transfer together with the band structure (and the buckling distortion of graphene layer presented above) indicates that interactions between the stacking sheets of the superlattice are more than just the van der Waals interactions.

4. Conclusions First-principles calculations based on density functional theory have been carried out to study the structural and electronic properties of superlattices composed of graphene and MoS2 monolayer. Our results for all the four possible stacking models considered revealed that graphene/MoS2 superlattices all exhibit metallic electronic properties, which are different from the isolated graphene and monolayer MoS2. A small band gap is opened up around the Fermi level at the K-point of the Brillouin zone for all the four structural models. The gap opened depends on the stacking geometry of graphene layers only. Besides, a small amount

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of charge transfer is found from the graphene layer to the intermediate region of C-S layers. Together with the band structures, small charge transfer indicates that the interaction between the stacking sheets of the superlattices are also weak ionic, i.e., more than just the van der Waals interactions. In addition, small buckling in the graphene layers also imply stronger interactions than the van der Waals one in the material.

Fig. 4 (Color online) Contour plots of the deformation charge density, ∆ρ1, of planes passing through (a) C-, (b) S- and (c) Mo-layers in the AC1BC2–stacking superlattice. (d) The charge density differences, ∆ρ2, on a plane perpendicular to the graphene and MoS2 layers, passing through Mo and two S atoms. (e) The deformation charge density, ∆ρ1, of a plane parallel to and 0.5 Å above the graphene layer (please refer to (f)). The grey, purple and yellow balls represent C, Mo and S atoms, respectively. Orange and blue lines correspond to ∆ρ > 0 and ∆ρ < 0, respectively.

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Acknowledgments This work is supported by the National 973 Program of China (Grant No. 2011CB935903) and the National Natural Science Foundation of China under grant No. 11004165, 21233004.

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Figure captions

Fig. 1 (Color online) (a) Top view of the two single MoS2 monolayer and the two graphene sheets used in the supercell; (b) Side view of the four different stacking models; and (c) Four different stacking configurations. C, Mo and S atoms are represented by grey, purple and yellow balls, respectively.

Fig. 2 (Color online) Band structures of the four superlattices with different stacking. Detailed band structures in the vicinity of band gap opening are inserted. Red dashed line represents the Fermi level.

Fig. 3 (Color online) (a) Band structures of isolated graphene and monolayer MoS2, respectively. (b) Contribution from graphene and monolayer MoS2 to the band structure of superlattice is shown with circles, in the left and right panels of the plot, respectively. (c) Density of states for the superlattice with AC1BC2-stacking. The Fermi level is set to be at 0 eV.

Fig. 4 (Color online) Contour plots of the deformation charge density, ∆ρ1, of planes passing through (a) C-, (b) S- and (c) Mo-layers in the AC1BC2–stacking superlattice. (d) The charge density differences, ∆ρ2, on a plane perpendicular to the graphene and MoS2 layers, passing through Mo and two S atoms. (e) The deformation charge density, ∆ρ1, of a plane parallel to and 0.5 Å above the graphene layer (please refer to (f)). The grey, purple and yellow balls represent C, Mo and S atoms, respectively. Orange and blue lines correspond to ∆ρ > 0 and ∆ρ < 0, respectively.

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Graphic for the TOC：：

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Structural and Electronic Properties of Superlattice Composed of

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