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Cite this:Phys. Chem. Chem. Phys., 2019,21, 22140

Tailoring the structural and electronic properties of an SnSe

₂

/MoS

₂

van der Waals heterostructure with an electric field and the insertion

of a graphene sheet†

Tuan V. Vu, ^abNguyen V. Hieu,^cLe T. P. Thao,^cNguyen N. Hieu, ^d Huynh V. Phuc,^eH. D. Bui,*^d M. Idrees,^f B. Amin, ^gLe M. Duc^hand Chuong V. Nguyen *^h

van der Waals heterostructures (vdWHs), obtained by vertically stacking diﬀerent two-dimensional (2D) layered materials are being considered intensively as potential materials for nanoelectronic and optoelectronic devices because they can show the most potential advantages of individual 2D materials.

Here, we construct the SnSe2/MoS2 vdWH and investigate its electronic and optical properties using first-principles calculations. We find that the band structures of both MoS2and SnSe2 monolayers are well kept in the SnSe2/MoS2 vdWH because of their weakly interacting features via vdW interaction.

The SnSe2/MoS2vdWH forms a type-I band alignment and exhibits an indirect semiconductor band gap of 0.45 eV. The type-I band alignment makes the SnSe2/MoS2 vdWH a promising material for optoelectronic nanodevices, such as light emitting diodes because of ultra-fast recombination of electrons and holes. Moreover, the band gap and band alignment of the SnSe2/MoS2 vdWH can be tailored by the electric field and the insertion of a graphene sheet. After applying an electric field, type-I to type-II and semiconductor to metal transitions can be achieved in the SnSe2/MoS2vdWH. Besides, when a graphene sheet is inserted into the SnSe2/MoS2vdWH to form three stacking types of G/SnSe2/ MoS2, SnSe2/G/MoS2 and SnSe2/MoS2/G, the p-type semiconductor of the SnSe2/MoS2 vdWH is converted to an n-type Ohmic contact. These findings provide theoretical guidance for designing future nanoelectronic and optoelectronic devices based on the SnSe2/MoS2vdWH.

1 Introduction

Following the successful exfoliation of single layered graphene (G) by Geim et al.,¹ a large amount of graphene-like two- dimensional (2D) materials have attracted worldwide interest because they can possess a lot of unique properties and

potential applications. Nowadays, the emerging 2D materials that can provide the fundamental suitable properties for novel high-performance nanodevices include hexagonal boron nitride (h-BN),² transition metal dichalcogenides TMDs,^3–5 and phosphorene analogues.^6,7 However, these 2D materials still have some drawbacks, which may in turn limit their practical applications. For instance, the lack of a valuable band gap in graphene prevents its application in optoelectronic devices. Whereas, the large band gap of h-BN limits its application in photocatalysis.⁸ Meanwhile, one major drawback of phosphorene is its air-stability, which restrains its use under open air.⁹All these disadvantages of 2D materials lead scientists to find common methods to break these limitations.

Recently, there have been many emerging approaches, that can be eﬀectively used to tune the physical properties of 2D materials, such as applying strain engineering^10–12and electric field,^13–15 and constructing van der Waals heterostructures (vdWHs).^16–18Among these, constructing vdWHs by vertically stacking different 2D materials on top of each other is known to be a powerful strategy to tune the electronic properties of 2D

aDivision of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam. E-mail: [email protected]

bFaculty of Electrical & Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam

cDepartment of Physics, University of Education, The University of Da Nang, Da Nang, Vietnam

dInstitute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam. E-mail: [email protected]

eDivision of Theoretical Physics, Dong Thap University, Cao Lanh 870000, Vietnam

fDepartment of Physics, Hazara University, Mansehra 21300, Pakistan

gDepartment of Physics, Abbottabad University of Science and Technology, Abbottabad 22010, Pakistan

hDepartment of Materials Science and Engineering, Le Quy Don Technical University, Ha Noi 100000, Vietnam. E-mail: [email protected]

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp04689e Received 23rd August 2019,

Accepted 23rd September 2019 DOI: 10.1039/c9cp04689e

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Published on 24 September 2019. Downloaded by Vanderbilt University Library on 1/2/2020 8:02:05 PM.

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materials and to expand their potential applications. To date, there is a large amount of vdWHs based on the different 2D materials, which have been experimentally synthesized and theoretically explored.^19–27 Owing to their promising extra- ordinary electronic and optical properties, these 2D vdWHs are being considered as building blocks for high-performance electronic and optoelectronic devices.^28–33 For instance, Cui et al.³⁰showed that the band edge positions in the MoS₂/g-GaN heterostructure are suitable for photocatalytic water splitting.

Li et al.³³ demonstrated that the SiC/MoS2 heterostructure possesses an excellent optical absorption, which merits its application for nanoelectronics and nanophotonics.

More recently, a member of the TMD family, MoS2, has received much more research interest owing to its high photo- luminescence quantum yield.³The MoS2monolayer has successfully been prepared in experiments by a mechanical exfoliation technique.³ It exhibits a direct band gap semiconductor, that makes it a suitable material for electronics and optoelectronics such as field-eﬀect transistors, integrated circuits and logic devices.^34,35To date, the combinations between MoS₂and other semiconductors have been fabricated experimentally and investigated theoretically.24,30,36–40For instance, Tongayet al.³⁶prepared the MoS₂/WS₂vdWHs from a CVD method. Whereas, the structural and electronic characteristics of MoS₂/MoSe₂ vdWH were determined by Kang et al.³⁷ from first principles calculations.

Luoet al.²⁴ studied the electronic and optical properties of the MoS2/Mg(OH)2 heterostructure and demonstrated that such heterostructure exhibits a type-II band alignment with an indirect semiconductor band gap. Very recently, Zhouet al.⁴¹successfully prepared the MoS2/SnSe2 vertical heterostructurevia a two-step chemical vapor deposition (CVD) method. They also demonstrated that the responsivity of the photodetector based on MoS2/SnSe2vdWH is enhanced up to 9.110³A W¹, which is higher than that in an MoS₂-based photodetector by 10³times.⁴² However, based on our best knowledge, there is no theoretical study on the combinations between MoS₂and SnSe₂monolayers to have a better understanding of the physical mechanism behind this.

In this work, we for the first time construct an MoS2/SnSe2

vdWH and investigate its structural and electronic properties as well as the eﬀect of external electric field using first principles calculations. Furthermore, it is interesting that with a high electron aﬃnity, the 1T phase of an SnSe2 monolayer, which has experimentally been synthesized by mechanical exfoliation,⁴³ can be considered to be a suitable material for the fabrication of Ohmic contacts. Thus, we also construct the combinations between G and MoS2/SnSe2 to form three stacking types of G/SnSe2/MoS2, SnSe2/G/MoS2and SnSe2/MoS2/G vdWHs.

2 Computational method

All our calculations in the present study have been carried out by using density functional theory (DFT) from the QUANTUM ESPRESSO package.⁴⁴The exchange–correlation potentials were described by the Perdew–Burke–Ernzerhof (PBE) functional

within the generalized gradient approximation (GGA) formalism.⁴⁵ Moreover, the DFT-D2 method based on the Grimme scheme⁴⁶ is employed to consider the vdW interactions between heterostructure layers. The energy cutoff for the plane-wave expansion is set to be 410 eV. All the geometric structures are fully relaxed until the energy and forces are converged to 10⁵ eV and 0.001 eV Å¹, respectively. The Brillouin zone integration is sampled with a 7 7 1 grid being used for all structural optimization and electronic property calculations. The vacuum space in thez-direction is about 30 Å to separate the interactions between neighboring slabs. Additionally, the calculations were performed using the hybrid Heyd–Scuseria–Ernzerhof (HSE) method to check the validity of the results given by the PBE method.

The optical absorption coeﬃcienta(o) can be calculated as follows:

aðoÞ ¼ ffiffiffi 2 p

o

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e₁²ðoÞ þe₂²ðoÞ q

e₁ðoÞ

1=2

; (1)

wheree₁(o) ande₂(o) are the real and imaginary parts of the dielectric function, respectively.

3 Results and discussion

3.1 Structural and electronic properties of SnSe2/MoS2vdWH Firstly, to construct the SnSe₂/MoS₂ vdWH, it is required to check the lattice parameters of the perfect SnSe₂ and MoS₂ monolayers, which are 3.81 Å and 3.18 Å, respectively. These values are consistent with available theoretical and experi- mental reports.^3,47It indicates that our calculations are reasonable. Moreover, both the monolayers of MoS₂ and SnSe₂ are known to be dynamically stable, which can be checked by calculating their phonon dispersion curves, as displayed in Fig. 1. Indeed, we can see clearly from Fig. 1 that the absence of imaginary frequencies in the phonon dispersions of MoS2

and SnSe2monolayers guarantees their dynamical stability. It is well known that the properties of SnSe2and MoS2monolayers are very sensitive to in-plane strains. Thus, in order to minimize the lattice mismatch between the stacking blocks, the SnSe2/ MoS2vdWH is built by ffiffiffi

p3 ffiffiffi

p3

of SnSe2cells and (22) of MoS2 cells. The lattice constant of the SnSe2/MoS2 vdWH is 6.48 Å, where the strain of 1.8% is distributed over both monolayers, compressive for the SnSe2 and tensile for the MoS2.

Fig. 1 Phonon dispersion curves of a MoS₂monolayer (a) and SnSe₂ monolayer (b) at the ground state.

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This strain distribution gives rise to small variations in the electronic properties of the heterostructure. Fig. 2 shows the atomic structure of SnSe2/MoS2 vdWH at the ground state.

It should be noted that there have been two kinds of stacking for 2D vdWHs, namely AA and AB stacking configurations.

Here, we built the SnSe2/MoS2 vdWH to minimize the lattice mismatch. Therefore, both AA and AB stacking are included in such a large supercell. Besides, we have also considered the SnSe2/MoS2vdWH with a 301twist angle by rotating the upper layer, but the calculated binding energy is much larger than that without rotation. It is well known that the lower the value of the binding energy is, the more stable the structure of the heterostructure is. Hence, we only choose the most stable structure as an object of concrete research in our manuscript.

After geometric optimization, we obtain the interlayer distance of D, which is 3.38 Å between two nearest Se and S layers in such vdWH. We can see that this value ofDis larger than the sum of the covalent radii of Se (1.2 Å) and S (1.05 Å) but is still smaller than their vdW radii. These predictions indicate that the interlayers are displaced and are not strictly on top of each other, thus, they are beyond the bonding range.

In addition, to confirm the structural stability of such vdWH, we further calculate its binding energy as follows:

Eb = [ESnSe2/MoS2 ESnSe2 EMoS2]/A, where ESnSe2/MoS2, ESnSe2

andEMoS2are the total energies of the SnSe2/MoS2vdWH, and isolated SnSe₂and MoS₂monolayers, respectively.Astands for the surface area of such heterostructures. Our obtainedE_bis 0.25 eV per [MoS₂] unit cell,i.e.,12.69 meV Å¹. It is clear that thisE_bvalue is the same magnitude as other vdWHs, such as PbI₂/MoS₂(E_b=0.234 eV per unit)⁴⁸but it is still lower than that in SnS2/PbI2 vdWH (Eb = 0.124 eV per unit).⁴⁹ These predictions demonstrate the weak vdW interactions in the SnSe2/MoS2vdWH.

The band structures of the isolated MoS2and SnSe2mono- layers as well as their SnSe2/MoS2 vdWH were also calculated using PBE and HSE06 methods, as displayed in Fig. 3 and Fig. S1 in the ESI.†We can see from Fig. 3(a) that the MoS2

monolayer exhibits a direct band gap semiconductor, in which both the valence band maximum (VB) and conduction band minimum (CB) are located directly at theMpoint. Whereas, the band structure of the SnSe₂monolayer in Fig. 3(b) shows an indirect band gap semiconductor, in which the CB locates at theGpoint and the VB lies along theM–Kpath. Our calculated direct and indirect band gaps of MoS2and SnSe2monolayers given by the PBE/HSE06 method are 1.70 eV/2.21 eV and 0.66 eV/1.26 eV, respectively. One should note that the more accurate HSE06 band structure can be basically obtained by upshifting/downshifting the conduction/valence bands of the PBE bands, indicating that the band dispersions given by the PBE functional are still reasonable though the bandgap is too small. Therefore, the electronic properties of the heterostructures under external electric field are calculated using the PBE method to obtain a changing trend. Fig. 3(c) displays the band structures of combined SnSe₂/MoS₂ vdWH, which exhibits a semiconducting behavior with an indirect band gap of 0.45 eV.

This band gap is still smaller than that of both MoS₂and SnSe₂ monolayers. In addition, we find that the band structures of both MoS2and SnSe2monolayers are well kept in the SnSe2/MoS2

vdWH because they weakly interact together via the vdW interaction. The SnSe2/MoS2 vdWH exhibits an indirect band Fig. 2 (a) Top view and (b) side view of the relaxed atomic structure of

SnSe2/MoS2vdWH.

Fig. 3 Band structures of (a) isolated MoS2, (b) SnSe2and (c) SnSe2/MoS2

vdWH. (d) Band edge positions of MoS2, SnSe2 monolayers, and the SnSe2/MoS2vdWH given by PBE and HSE calculations.

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gap semiconductor with the band gap of 0.45 eV at the ground state. Interestingly, one can observe that both the CB and VB of the SnSe₂/MoS₂ vdWH come from those of the SnSe₂ monolayer, indicating a type-I band alignment. Such type-I band alignment in SnSe₂/MoS₂vdWH will make it become a suitable material for applications in high-performance optoelectronic nanodevices. Fig. 3(d) displays the band alignment in MoS₂, SnSe₂ monolayers and their vdWH given by PBE and HSE06 methods. It is clear that the band edge positions, including the CB and VB of the SnSe2 layer, are located below/above as compared with those of MoS2layers, confirming a type-I band alignment in such SnSe2/MoS2 vdWH. The shift of the band edge positions of the SnSe2 and MoS2 monolayers in SnSe2/ MoS2 vdWH may come from additional processes occurring at the interface including charge transfer, charge redistribution and interfacial dipoles. These phenomena were also experimentally observed in other vdWHs, such as the MoS2/WS2

vdWH,³⁶ and MoSe2/WS2 vdWH.⁵⁰ The type-I in the SnSe2/ MoS₂vdWH makes it a promising material for optoelectronic nanodevices, such as light-emitting diodes because of an ultra- fast recombination of electrons and holes.

We display in Fig. 4(a and b) the charge density diﬀerence and electrostatic potential of such vdWH. It is clear that the charge transfer and redistribution in such vdWH can be visualized in more detail by analyzing its charge density diﬀerence, which is calculated as follows:Dr=r_vdWHr_SnSe

2r_MoS

2, where r_vdWH,r_SnSe

2, andr_MoS

2, respectively, are the charge densities of the vdWH, SnSe2and MoS2monolayers. The charge density diﬀerence in the vdWH is illustrated in Fig. 4(a). Fig. 4(b) illustrates the diﬀerence in electrostatic potential between SnSe2 and MoS2

monolayers in their SnSe2/MoS2 vdWH. We find that the SnSe2

has a deeper potential than the MoS2, confirming that the charge is flowed from MoS2to SnSe2in such vdWH.

We next investigate the optical properties of the SnSe₂/MoS₂ vdWH by calculating its optical absorption coeﬃcient. The optical absorption coeﬃcients of the SnSe₂ and MoS₂monolayers are also plotted for comparison. These results are displayed in Fig. 5. We find that the SnSe₂, MoS₂and their SnSe₂/ MoS2vdWH show the absorption peaks in the region of visible

light. In addition, we can see that the SnSe2/MoS2vdWH has a wider absorption range than those of the SnSe2 and MoS2

monolayers. The absorption intensity of the SnSe2/MoS2vdWH can reach the order of about 10⁵cm¹at the ground state.

3.2 Eﬀects of electric field

Now, an interesting question that relates to the potential applications of the SnSe2/MoS2 vdWH as a component of high- performance nanoelectronic devices, is whether the electronic properties of such vdWH can be modulated when it is subjected to an external electric field (E>). Therefore, we further consider the impact of theE>on the electronic properties of the SnSe2/ MoS₂vdWH. In the SnSe₂/MoS₂vdWH, the positive direction of theE>is directed from the MoS₂layer to the SnSe₂layer, as displayed in Fig. 6. We can see from Fig. 6(b) that the band gap of the SnSe₂/MoS₂vdWH varies when it is subjected to theE>. The band gap of such vdWH, in particular, decreases by applyingE>. For a positiveE>, the band gap decreases from 0.45 eV at theE>= 0 V Å¹to 0.07 eV at theE>= +0.4 V Å¹. This indicates that the SnSe2/MoS2 vdWH retains a semiconducting character. However, when the positiveE>is continuously increased,i.e.,E>4+0.4 V Å¹, the band gap of such vdWH tends to decrease and becomes zero, indicating that the vdWH gains metallic character. For a negative E>, the band gap of the vdWH also linearly decreases from 0.45 eV at the E>= 0 V Å¹to 0.20 eV at theE>=0.3 V Å¹. Interestingly, when the negative E> is further decreased, the band gap of such vdWH tends to a continuous decrease and becomes zero at the E>= 0.4 V Å¹. The SnSe₂/MoS₂vdWH, in this case, demonstrates a metallic behavior, i.e., the semiconductor-to- metal transition can be achieved in such vdWH by applyingE>. It should be noted that the diﬀerence in the electronegativity between Se and S layers in the SnSe2/MoS2vdWH may cause to spontaneous electric polarization, leading to a decrease in its band gap withE>.

Fig. 4 (a) Charge density diﬀerence and (b) electrostatic potential of the SnSe₂/MoS₂vdWH at the ground state.

Fig. 5 Optical absorption coeﬃcient of the SnSe2, MoS2monolayers and their SnSe2/MoS2vdWH.

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To have a clear understanding, we also calculate the band edge positions, which mainly contributed by SnSe₂ and MoS₂ parts for such vdWH. The diﬀerence between the CB and VB of the SnSe₂/MoS₂and the Fermi level is denoted byD^SnSe_CB ².

D^MoS_CB ² andD^SnSe_VB ².

D^MoS_VB ², respectively, as displayed in Fig. 6(c). Addi- tionally, the band structures of the SnSe2/MoS2vdWH are also calculated and displayed in Fig. 7. One can find that when a negativeE>is applied, the CB of the SnSe2layer shifts downward to the Fermi level, whereas the CB of the MoS2layer shifts upward far from the Fermi level. These findings lead to a/an decrease/increase in theD^SnSe_CB ².

D^MoS_CB ², as illustrated in Fig. 6(c) and 7(a). In contrast, the VB of both SnSe₂ and MoS₂ layers shifts upward to the Fermi level. With negativeE>, one can observe that the upshift to the Fermi level of the VB of the MoS2

layer is faster than that of the SnSe2layer. When the negative E> reaches 0.3 V Å¹, the VB of the MoS2 becomes higher than that of the SnSe2layer. Thus, in this case, the band gap of the SnSe2/MoS2 vdWH is mainly contributed by the CB of the SnSe2layer and the VB of the MoS2 layer, forming a type- II band alignment. More interestingly, when the negativeE>is continuously decreased to0.4 V Å¹, both the CB and VB of

the SnSe₂/MoS₂ vdWH cross the Fermi level. The SnSe₂/MoS₂ vdWH demonstrates a metallic character. Therefore, the electronic properties of the SnSe₂/MoS₂ vdWH can be turned with negativeE>. The conversion from type-I to type-II band alignment is observed atE>=0.3 V Å, whereas the transition from semiconductor to metal can be achieved in such vdWH at E>=0.4 V Å¹.

The changes in the band structures of the SnSe2/MoS2vdWH when it is subjected to positiveE>are illustrated in Fig. 7(b).

We can see that the CB of both the SnSe2and MoS2layers move downward to the Fermi level, resulting in a decrease of the D^SnSe_CB ² andD^MoS_CB ², as displayed in Fig. 6(c). In contrast, in the case of the positive E>, the VB of the SnSe₂ layer tends to upshift to the Fermi level, resulting in a decrease in theD^SnSe_VB ², while the VB of the MoS2layer moves downward far from the Fermi level, leading to an increase in theD^MoS_VB ². Interestingly, we find that when the positiveE>is larger than +0.4 V Å¹, the VB of the SnSe2layer shifts upward and crosses the Fermi level, resulting in a transition from semiconductor to metal in such vdWH. The controllable electronic properties of the SnSe2/MoS2 vdWH via externalE>make it become a potential material for applications in high performance optoelectronic and nanoelectronic devices.

Fig. 6 (a) Schematic diagram of applied electric field along thezdirection of the SnSe₂/MoS₂vdWH. (b) The variation of the band gap of the SnSe₂/MoS₂ vdWH as a function ofE>. (c) The dependence of the band edges contributed by the SnSe2and MoS2monolayers in its vdWH as a function ofE>.

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3.3 The insertion of a graphene layer into the SnSe₂/MoS₂vdWH What is more, it is interesting that the SnSe₂can be considered as a good candidate for realizing an Ohmic contact. Thus, we

further consider the eﬀect of a graphene layer, inserted into the SnSe₂/MoS₂vdWH to form three trilayer stacking configurations of G/SnSe₂/MoS₂, SnSe₂/G/MoS₂and SnSe₂/MoS₂/G. These forming Fig. 7 Energy band structures of the SnSe₂/MoS₂vdWH under diﬀerent negative (a) and positive (b) electric fields.

Fig. 8 (a) Top view and (b) side view of the relaxed atomic structure of the G/SnSe₂/MoS₂, SnSe₂/G/MoS₂and SnSe₂/MoS₂/G vdWHs (from left to right).

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stacking configurations are displayed in Fig. 8. The G–SnSe2, SnSe2–MoS2and MoS2–G interlayer distances denote by theD1,D andD2, respectively. Moreover, to check the structural stability of such vdWHs, we further calculate its binding energy as follows:

Eb = [EH EG ESnSe₂ EMoS₂]/A², where EH,EG,ESnSe₂, and EMoS₂, respectively, are the total energies of the vdWHs formed by graphene, SnSe2and MoS2monolayers.Ais the surface area of the vdWH. Our calculated interlayer distances and binding energies are listed in Table 1. One can observe from the obtained interlayer distances and binding energy that in these combined heterostructures, graphene, SnSe₂ and MoS₂ monolayers are bondedviaweak vdW forces.

The electronic band structures of these stacking configurations of the G/SnSe₂/MoS₂, SnSe₂/G/MoS₂ and SnSe₂/MoS₂/G vdWHs are displayed in Fig. 9(a–c). We firstly find that the intrinsic electronic band structure of graphene is well preserved

in all these vdWHs. It demonstrates that all key advantages of the electronic properties of perfect graphene can be conserved in these vdWHs. In addition, we can see that a tiny band gap of graphene in the G/SnSe₂/MoS₂, SnSe₂/G/MoS₂, and SnSe₂/MoS₂/G vdWHs is opened owing to the sublattice symmetry breaking.

These opened band gaps of graphene are listed in Table 1.

However, we find that these values are still lower than the thermal fluctuation of 26 meV at room temperature. Thus, they can be considered negligible.

More interestingly, when the graphene layer is added to the SnSe2/MoS2vdWH, it results in the formation of the metal–

semiconductor contact. According to the Schottky–Mott rule, the Schottky or Ohmic contact can be formed in these G/SnSe2/ MoS2, SnSe2/G/MoS2, and SnSe2/MoS2/G vdWHs. The schematic illustration of the Schottky and Ohmic contact types, forming at the metal–semiconductor contact is displayed in Fig. 9(d).

We can see from the electronic band structures of these G/SnSe2/MoS2, SnSe2/G/MoS2 and SnSe2/MoS2/G vdWHs, as displayed in Fig. 9, that they form the n-type Ohmic contact (nOC-type). Moreover, in order to confirm the Ohmic contact in these heterostructures, HSE calculations are also employed to calculate their band structures for the energetically most stable configuration, i.e., SnSe₂/G/MoS₂ vdWH, as shown in Fig. S2 in the ESI.†It is clear that the SnSe₂/G/MoS₂ vdWH exhibits Ohmic contact. The barrier height of the p-type Schottky contact Table 1 Calculated interlayer distances [in Å], band gap opened in

graphene [in meV], and the binding energy [in meV Å²] of the considered heterostructures

Systems D₁ D₂ D E_b E_g

G/SnSe₂/MoS₂ 3.332 10.040 3.382 29.44 2.8 SnSe₂/G/MoS₂ 3.339 3.279 7.618 32.37 8.2 SnSe₂/MoS₂/G 9.849 3.341 3.357 28.43 4.0

Fig. 9 (a–c) Projected electronic band structures of the G/SnSe₂/MoS₂, SnSe₂/G/MoS₂and SnSe₂/MoS₂/G vdWHs. Red, blue and green lines represent the contributions of graphene, MoS₂and SnSe₂monolayers in these heterostructures, respectively. (d) The schematic description of the Schottky and Ohmic contact types, forming at the metal–semiconductor contact.

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in these G/SnSe2/MoS2, SnSe2/G/MoS2, and SnSe2/MoS2/G vdWHs is small at about 0.50 eV. The transformation from a p-type semiconductor in the SnSe₂/MoS₂vdWH to nOC-type when the graphene layer is added makes these vdWHs potential candidates for future nanodevices.

4 Conclusions

In summary, we have investigated systematically the structural, electronic and optical properties of the SnSe2/MoS2 vdWH using first-principles calculations. Our results demonstrate that the SnSe₂/MoS₂ vdWH forms a type-I band alignment and exhibits an indirect band gap semiconductor at the ground state. Both the band alignment and band gap of the SnSe₂/MoS₂ vdWH can be controlled by applying an electric field or by inserting a graphene sheet. When the electric field is applied, the type-I band alignment can be transformed to the type-II one and the transition from semiconductor to metal is also achieved in the SnSe2/MoS2 vdWH. In addition, when a graphene sheet is inserted into the SnSe2/MoS2 vdWH to form three stacking types of the G/SnSe2/MoS2, SnSe2/G/MoS2, and SnSe2/MoS2/G, the p-type semiconductor of the SnSe2/MoS2

vdWH is converted to an n-type Ohmic contact. These findings provide theoretical guidance for designing future nanoelectronic and optoelectronic devices based on the SnSe₂/MoS vdWH.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.01-2019.05. B. Amin acknowledges support from the Higher Education Commission of Pakistan (HEC) under Project No. 5727/KPK/NRPU/R&D/HEC2016.

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