Realizing semiconductor to metal transition in graphitic ZnO and MoS₂ nanocomposite with external electric field

Abstract

First-principles calculations have been used to investigate the structural and electronic properties of graphitic ZnO and MoS₂ (g-ZnO/MoS₂) nanocomposites. It is found that the binding strength of g-ZnO/MoS₂ exhibits strong dependence of atomic arrangement of g-ZnO relative to MoS₂. The coupling interaction of g-ZnO/MoS₂ obviously reduces the semiconducting band gaps, compared to both individual sheets, which are sensitive to its stacking orders. Interestingly, the vertical external electric field (E-field) can be applied to enhance the stability of g-ZnO/MoS₂ and increase charge transfers between these two component. Furthermore, the E-field with the positive direction from MoS₂ to g-ZnO can tune the band gap of g-ZnO/MoS₂ nanocomposites, whereas this nanocomposites produce the semiconducting to metallic behavior transitions when the E-field changes from positive to negative direction, regardless of the stacking pattern. The tunable electronic properties of g-ZnO/MoS₂ nanocomposites under the E-field are attributed to the changes in electrostatic potential difference between atom layer of MoS₂ and interlayer region close to g-ZnO. Present results suggest that the g-ZnO/MoS₂ heterojunction provides promising applications for MoS₂-based optoelectronic and nanoelectronic devices, such as fabricating field effect transistor (FET).

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1. Introduction

Two dimensional (2D) nanomaterials including graphene^1–3 and other graphene-like materials⁴ have attracted great attention due to their unusual properties and wide potential applications. As important 2D materials, transition metal dichalcogenides (TMDs), such as MoS₂, WS₂, and MoSe₂, are extensively investigated, both experimentally and theoretically, because of their distinctive electronic,^5–9 optical,^10,11 and catalytic properties in hydrogen evolution reaction.^12–16 Monolayer MoS₂ possesses a direct band gap of 1.9 eV, which well complements semimetallic graphene and insulating hexagonal boron nitride (h-BN), but the band gap becomes indirect for bulk MoS₂ (1.2 eV).¹⁷ In particular, by combining the electronic properties of graphene and MoS₂, Roy et al.¹⁸ have fabricated the most sensitive graphene-based photodetetor that shows remarkable dual optoelectronic functionality. Owing to the presence of intrinsic defects, engineering the electronic structures of MoS₂ is important for the practical applications in many fields.¹⁹

Among several strategies^5,20–26 being employed to manipulate the electronic properties of TMDs materials, combining the MoS₂ and other 2D nanomaterials by the weak interaction, such as WS₂,²² MoSe₂,^23,24 h-BN,²⁵ and graphene,^18,26 is a particularly interesting one. First-principles density functional (DFT) calculations²² show that the MoS₂–WS₂ heterojunction has an optically active band gap, smaller than that of the corresponding single components. Owing to the interface interaction, bilayer TMDs show a linear reduction of the band gap under external electric fields, while the band gap in monlayer TMDs are unaffected by such fields.²⁷ Using DFT calculations, Gillen et al. showed that the n doping of h-BN substrates by oxygen, carbon, and sulfur impurities, other than p doping, results in noticeable charge transfer into the conduction band structure of MoS₂.²⁵ When graphene is supported on the monolayer MoS₂, the hybrid structures can enhance electrochemical performance for lithium-ion batteries, and intrinsic electronic properties of graphene are preserved by the MoS₂ substrates.^28–31 Very recently, Musso et al.³² reported that graphene oxides could be used as an efficient hole injection layer for monolayer MoS₂-based electronic and optoelectronic devices by means of DFT calculations. Therefore, it would be very interesting to fabricate and study new 2D MoS₂-based hybrid structures with tunable electronic properties.

ZnO is a promising optoelectronic materials, which is widely used in the field emission fields³³ and other related fields³⁴ due to its wide band gap of 3.44 eV.³⁵ The combination of ZnO with MoS₂ nanoflowers can enhance the photocatalytic and field emission properties of pure MoS₂.³⁶ Recently, the DFT calculations³⁷ revealed that the conductivity of MoS₂ can be tuned from n- to p-type conducting, depending on the microstructure of substrates ZnO (0001). In fact, the ZnO, in the form of graphitic honeycomb structure, is preferentially formed than ZnO (0001) structure when the layer number is reduced as revealed by experimental³⁸ and theoretical works.^39–42 Many interesting electronic and magnetic properties are found in 2D graphitic ZnO (g-ZnO) monolayer. Furthermore, Hu et al. reported⁴³ that the doped g-ZnO by Al or Li can induce substantial electron and hole doping in graphene as this two 2D materials form nanocomposites.

Motivated by these studies, the g-ZnO/MoS₂ heterojunction may present the exciting properties, qualitatively different from the individual component. Herein, using the first-principles calculations, we investigated the electronic structures of g-ZnO/MoS₂ hybrid structures with and without the external electric field. We found that the band gaps of g-ZnO/MoS₂ are tunable under the external electric field along the direction normal to the sheet from MoS₂ to g-ZnO, whereas the reverse electric field realizes the transition from semiconducting to metallic behavior, which may provide an effective way for fabricating MoS₂-based field effect transistor (FET).

2. Computational details

All calculations for g-ZnO/MoS₂ hybrid structures were performed within the framework of the plane-wave pseudopotential density-functional theory (DFT) implemented in CASTEP.⁴⁴ The Perdew–Burke–Ernzerhof (PBE)⁴⁵ functional with vdW correction proposed by Grimme⁴⁶ (DFT-D2) was used to deal with the exchange and correlation term due to its good description of long-range vdW interaction. The ultrasoft pseudopotentials⁴⁷ for the ion–electron interactions and a kinetic energy cutoff of 350 eV in the plane-wave expansion were used in calculations. The two-dimensional (2D) periodic boundary conditions are considered along the growth directions of hybrid structures between MoS₂ and g-ZnO. To separate the interaction between neighboring slabs, the vacuum space in the direction normal to the interface is set to 12 Å. The 2D Brillouin zone is sampled with 5 × 5 × 1 k-points within the Monkhorst–Pack scheme⁴⁸ for structural optimization and 11 × 11 × 1 k-points for electronic structural calculations. The whole configuration was allowed to relax until the convergence criterial of energy and force are less than 10⁻⁵ eV and 0.01 eV Å⁻¹, respectively.

When effect of the external electric field on electric structure of hybrid systems was discussed, the Perdew–Burke–Ernzerhof (PBE) functional with vdW correction proposed by Grimme and the double numerical plus polarization function (DNP) basis set, which are implemented in the DMOL3 package,⁴⁹ were used. Other calculated parameters are the same as the ones used in CASTEP. The dipole correction to the binding energy of g-ZnO/MoS₂ shows no noticeable influence with less than 5 meV, which is used to cancel the errors caused by periodic boundary condition. The DFT implementation in DMOL3 may be prone to significant Basis Set Superposition Error (BSSE). Using the counterpoise correction suggested by Boys and Bernaedi,⁵⁰ we estimate the BSSE correction to binding energies of relaxed structure for selected systems. The calculations show that the BSSE correction to binding energy has less influence on the predicted tendency for stability of g-ZnO/MoS₂.

The calculated lattice constants for g-ZnO and MoS₂ monolayer are 3.29 and 3.16 Å, respectively, which are well agreement with previous reports.^7,51,52 Only the MoS₂ with 2H phase, which represents hexagonal symmetry of the crystal structure, is discussed. We simulate the g-ZnO/MoS₂ hybrid structure by putting a 2 × 2 supercell of monolayer g-ZnO on top of a 2 × 2 supercell of MoS₂, which includes the lattice mismatch of less than 4%. To bring focus on the fundamental properties of MoS₂ in hybrid structure, we chose the lateral lattice parameter of a = b = 6.32 Å, which was optimized for isolated MoS₂. In fact, we also found that when MoS₂ is set to lattice match g-ZnO monolayer, no noticeable change in the main conclusions is obtained. Many previous calculations on graphene-based hybrid systems^29,53–55 have revealed such a negligible effect of the small lattice mismatch on the electronic structures. The formation of lattice-matched structure depends on the competition between interlayer binding energy and strain energy.⁵⁶

By changing the initial positions of the g-ZnO relative to MoS₂ sheets, the hybrid structures include five stacking patterns: one top and four hollow configurations. In the case of top pattern, the Zn atoms of g-ZnO are right above the S atom of MoS₂, and thus the O atom directly points to the Mo atom (Fig. 1a), namely as AA. For the hollow configuration, the Zn or O atoms of g-ZnO could be placed directly at the hexagonal ring center of MoS₂ consisting of three nearest-neighboring S and Mo atoms. Therefore, each hollow configuration contains two stacking patterns: the remaining O or Zn atoms in g-ZnO can directly point to the Mo or S atom, namely as AB-Zn (or AB-Zn-2) and AB-O (or AB-O-2) as shown in Fig. 1b–e, respectively. The effect of stacking patterns on electronic properties of g-ZnO/MoS₂ hybrid structures will be discussed.

Fig. 1 Top and side view of geometrical structures of g-ZnO/MoS₂ with (a) AA, (b) AB-Zn, (c) AB-Zn-2, (d) AB-O, and (e) AB-O-2 stacking patterns. The distance is in Å.

To evaluate the stability of hybrid g-ZnO/MoS₂, the binding energies (E_b) are calculated by E_b = E_g-ZnO/MoS2 − E_MoS2 − E_g-ZnO, where E_MoS2, E_g-ZnO, and E_g-ZnO/MoS2 are the total energies of isolated MoS₂, g-ZnO, and g-ZnO/MoS₂ heterojunction, respectively. Note that the negative binding energy shows energetically favorable g-ZnO/MoS₂ heterojunction.

3. Results and discussion

3.1. Structural properties and stabilities of g-ZnO/MoS₂

We first investigate the structural properties of g-ZnO supported on monolayer MoS₂. Fig. 1a–e show the geometrical structures of g-ZnO/MoS₂ heterojunction. Table 1 presents the calculated equilibrium interlayer distances and binding energies for g-ZnO/MoS₂ hybrid structure. The g-ZnO remains the same basal plane structure as its individual sheet as it is stacked on monolayer MoS₂ for all five configurations with lattice parameter of isolated g-ZnO after geometry relaxation. The Zn–O bonds in AB-Zn (or O)-2 configuration exhibit slight distortion relative to other configurations (see Fig. 1c and e). The interlayer spacing for g-ZnO/MoS₂ hybrid structure is evaluated to be about 3.0 Å regardless of the stacking pattern, which belongs to a typical vdW equilibrium spacing. Such distance is comparable with recent theoretical results on other MoS₂-based nanocomposites, such as graphene/MoS₂ (ref. 29) and h-BN/MoS₂.²⁵

Table 1 Summary of calculated results for g-ZnO/MoS₂ with different stacking patterns and with monolayer (1) and bilayer (2) MoS₂: the binding energy per unit cell (E_b in meV), interlayer distance (Δd in Å), charge transfer per unit cell from g-ZnO to MoS₂ (ΔQ in e), and band gap (E_g in eV)

Configuration	MoS2	Eb		Δd		ΔQ		Eg
		C1^a	C2^b	C1	C2	C1	C2	C1	C2
a The lattice constant of g-ZnO/MoS2 is set to that of isolated g-ZnO.b The lattice constant of g-ZnO/MoS2 is set to that of isolated MoS2.
AA	1	−108	−28	3.07	3.04	−0.02	−0.02	0.25	0.57
AB-Zn	1	−65	7	3.06	3.02	0	0	0.76	1.03
AB-Zn-2	1	−163	−95	3.04	3.01	−0.01	−0.01	0.55	0.8
AB-O	1	−162	−81	3.03	3	0	−0.01	0.56	0.78
AB-O-2	1	−115	−33	3.06	3.03	−0.02	−0.01	0.28	0.51
AA	2	−57	−21	2.92	2.89	−0.02	−0.02	0.18	0.51
AB-Zn	2	10	61	2.96	2.92	0	0	0.39	0.96
AB-O	2	−75	−88	3.05	2.83	0	0	0.27	0.86

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As shown in Table 1, the formation of g-ZnO/MoS₂ hybrid structure with lattice parameter of isolated g-ZnO for all stacking patterns is more favorable energetically than that with lattice parameter of isolated of MoS₂ due to the constrained g-ZnO. The stability of g-ZnO/MoS₂ hybrid structure is sensitive to the stacking configurations. The AB-O- or AB-Zn-2-stacked nanocomposite with lattice constant of isolated g-ZnO has the highest binding energy of −162 meV per unit cell among five configurations. Other three configurations, AA, AB-Zn, and AB-O-2, have the favorable binding energies of −65 to −115 meV per unit cell. The stability of g-ZnO/MoS₂ hybrid structures can be comparable to a large number of layered compounds.^{25,29,43,57–59}

As an benchmark, the PBE-D2 calculations show that the distance between metal layer in isolated MoS₂ bilayer is 6.05 Å, which fully agree with previous experimental and theoretical values.^27,60 Other accurate van-der Waals exchange–correlation functionals are also applied to study graphene- or MoS₂-based heterojunctions with weak coupling interaction.⁶¹ Thus, we also performed the calculations on g-ZnO/MoS₂ by using the vdW-DF functional.⁶² The predicted binding energies of g-ZnO/MoS₂ for five configurations by vdW-DF are −127 to −201 meV per unit cell, larger than that of corresponding configuration by DFT-D2. This results suggest that the improved stability of hybrid structure by vdW-DF is dependent on the stacking pattern. The interlayer distances of 3.05–3.14 Å between g-ZnO and MoS₂ are comparable with the values by PBE-D2. Thus, the predicted tendencies for coupling of g-ZnO with MoS₂ via vdW interaction are consistent by both functionals.

Because the semiconducting properties of MoS₂ with direct or indirect band gap depend on its thickness, herein, the structural and electronic properties of g-ZnO supported on bilayer MoS₂, namely as g-ZnO/two-MoS₂, are also investigated. Fig. S1 shown in the ESI† present the geometrical structures of g-ZnO/two-MoS₂ heterojunction. As shown in Table 1, the typical vdW equilibrium spacing between g-ZnO and its neighbor MoS₂ layer is still obtained with 2.92–3.05 Å. However, the bilayer MoS₂ reduces the binding strength of g-ZnO/MoS₂.

3.2. Electronic properties of g-ZnO/MoS₂

We now discuss the electronic properties of g-ZnO/MoS₂ nanocomposites. The band structures of isolated g-ZnO and MoS₂ monolayer and three hybrid g-ZnO/MoS₂ structures with lattice parameter of isolated MoS₂ are shown in Fig. 2a, b and 3a–c (see Fig. S2† for other stacking and Fig. S3† for hybrid with other lattice parameter), respectively. Monolayer g-ZnO and MoS₂ sheets are semiconducting with a direct band gap of 1.68 and 1.74 eV, respectively, which agree well with previous theoretical studies.^17,43,52,63 It is well known that GGA-PBE calculations always underestimate the band gap, while the screened hybrid HSE06 functional⁶⁴ typically predicts more accurate band gaps close to experimental data. The HSE06 calculations have given the band gaps of 3.2 and 2.01 eV for monolayer g-ZnO⁵² and MoS₂,^22,23 respectively. The Mulliken population analysis shows that the hybridization of g-ZnO with MoS₂ monolayer leads to small or a negligible charge transfer, further supporting the vdW interaction mechanism for g-ZnO/MoS₂ heterojunction.

Fig. 2 Band structure (a and b) and TDOS (c and d) of isolated g-ZnO and MoS₂ sheets. The Fermi level is set to 0.

Fig. 3 (a–c) Band structures, (d–f) partial charge densities of VBM (left panel) and CBM (right panel), and (g–i) TDOS and PDOS of g-ZnO and MoS₂ for g-ZnO/MoS₂ with (a), (d) and (g) AA, (b), (e), and (h) AB-Zn, and (c), (f), and (i) AB-O stacking patterns. The isosurfaces in (d–f) are 0.04 e Å⁻³, and the Fermi level is set to 0.

Comparing with the individual sheets, clearly, the g-ZnO/MoS₂ nanocomposites are still semiconductors, but the band gaps become indirect and reduce due to the interface interaction, which are sensitive to the stacking pattern. The AB-Zn-stacked nanocomposite with lattice parameter of isolated g-ZnO (MoS₂) possesses the widest band gap of 0.76 (1.03) eV, while the band gaps are 0.25 (0.57), 0.55 (0.8), 0.56 (0.78), and 0.28 (0.51) eV for AA, AB-Zn-2, AB-O, and AB-O-2 configurations, respectively (see Table 1). This indicates that changing the lattice constant of g-ZnO/MoS₂ can significantly tune the band gap. Such results are consistent with previous reports on electronic properties of MoS₂ sheet, in which the band gap of MoS₂ is sensitive to the tensile strain changing the lattice constant.⁶⁵ The reduced band gaps indicate that the photogenerated electrons transfer from the valence band maximum (VBM) to conduction band minimum (CBM) of g-ZnO/MoS₂ composites becomes easier, compared to the individual sheet. The stacking configurations-dependent band gap of g-ZnO/MoS₂ is different from other MoS₂-based heterojunctions, in which the stacking pattern has less influence on the band gap.^25,29

We also performed HSE06 calculations on g-ZnO/MoS₂ with AA and AB-O-2 stacking pattern as shown in Fig. S4.† The corresponding band gaps are predicted to be 1.45 and 1.5 eV, obviously smaller than that of both g-ZnO and MoS₂ sheets. This indicates that the reduction of band gap of g-ZnO/MoS₂ is independent of the employed functional. The partial charge densities (Fig. 3d–f and S2c, d†) show that the band gaps of g-ZnO/MoS₂ are mostly controlled by the p orbital of O atoms of g-ZnO and d_z² orbital of Mo of MoS₂, which contribute to the VBM located at Γ and CBM at K point, respectively.

To further understand the electronic properties of g-ZnO/MoS₂, we calculated the total density of states (TDOS) and the projected density of states (PDOS) of g-ZnO and MoS₂ as shown in Fig. 3g–i and S2e, f.† The TDOS calculations also show that the reduction of semiconducting band gap of g-ZnO/MoS₂ depends on the stacking configuration. Based on the PDOS, the VBM of g-ZnO/MoS₂ only consists of electronic states from g-ZnO, whereas the CBM is mainly contributed by the electronic states from MoS₂, which is consistent with the partial charge densities. The different contribution to VBM and CBM from two components demonstrates that a type-II heterojunction is formed, which is an important factor for the superior photoactivity of the MoS₂-based nanocomposites. The band structure (Fig. 4a and b) and DOS (Fig. 4c and d) calculations by HSE06 also show that the g-ZnO/MoS₂ hybrid belongs to type-II semiconductor.

Fig. 4 Charge density difference of g-ZnO/MoS₂ without and with E-field. (a) AA, (b) AB-Zn, and AB-O stacking patterns (c) without E-field and (d–f) with E-field. Blue and yellow areas denote electron accumulation and depletion, respectively, and the isosurfaces are 0.003 e Å⁻³.

By comparing with the band structures and PDOS of MoS₂ and g-ZnO before (Fig. 2) and after (Fig. 3) stacking, it is found that the positions of CBM of MoS₂ with respect to the Fermi level are slightly shifted downward after the interface interaction of g-ZnO, while many inherent electronic properties of MoS₂ are well preserved. On the other hand, owing to the MoS₂ contact, the VBM of g-ZnO is obviously shifted upward relative to the Fermi level, whose position is significantly higher than that of MoS₂, while the CBM is less changed, leading to reduction of band gap of g-ZnO. As a result, the electron transfers from VBM to CBM of g-ZnO become more effective, and these electrons can spontaneously transfer to CBM of MoS₂ because its energy level is about 0.25 eV lower than that of g-ZnO. These results confirm that the g-ZnO may potentially serve as donor materials in g-ZnO/MoS₂. We also calculated the electronic structures of g-ZnO/two-MoS₂ as shown in Fig. S5† and Table 1. The results show that the interlayer interaction between g-ZnO and bilayer MoS₂ also reduces the band gap.

To understand the charge transfer mechanism between g-ZnO and MoS₂, the charge density difference, Δρ = ρ_g-ZnO/MoS2 − ρ_g-ZnO − ρ_MoS2, is calculated as shown in Fig. 4, where ρ_g-ZnO/MoS2, ρ_g-ZnO, and ρ_MoS2 represent the total charge densities of g-ZnO/MoS₂ hybrid systems, free g-ZnO, and free MoS₂, respectively. Obviously, one can find that a significant charge accumulation is observed in the interlayer region for g-ZnO/MoS₂ with AA (Fig. 4a) and AB-Zn (Fig. 4b) configurations, while the negligible charge transfer between g-ZnO and MoS₂ for AB-O is found (Fig. 4c). Comparing with Fig. 4a and b, the charge redistribution along the vertical direction of g-ZnO and MoS₂ sheets displays distinct difference between AA and AB-Zn configurations. The electron accumulation for AA configuration appears on the interlayer regions, whereas this regions for AB-Zn exhibit electron depletion.

To further understand the reduction of band gap of g-ZnO/MoS₂, we examined the electrostatic potential profile. The planar averaged electrostatic potential of g-ZnO/MoS₂ with different stacking as a function of position in the z-direction is depicted in Fig. 5a. Clearly, a significant electrostatic potential difference between the layers of g-ZnO and MoS₂ is found, which leads to the built-in potential, giving rise to reduction of band gap of g-ZnO/MoS₂. The variations of xy-averaged electrostatic potential of g-ZnO/MoS₂ for all configurations with the z direction are the same each other, however, small difference between the five stacking patterns can be found, for example, the electrostatic potential in g-ZnO atom layer depends on its stacking pattern. In addition, we also calculated the dipole moments of g-ZnO/MoS₂ with different stacking patterns in the direction perpendicular to g-ZnO and MoS₂ sheets. The dipole moments of AA and AB-O-2-stacked structures are 0.012 and 0.022 e Å⁻¹, respectively, while this value is 0.0034 e Å⁻¹ for AB-O-1 pattern, and even a negligible dipole moment is found for AB-Zn. These results suggest that the band gap of g-ZnO/MoS₂ depending on the configurations is correlated with its dipole moment.

Fig. 5 The xy-averaged electrostatic potential of g-ZnO/MoS₂ with (a) five stacking patterns without the E-field and (b) initial AB-Zn under the E-field with different strength as a function of position in the z-direction.

3.3. Interaction of g-ZnO with MoS₂ under external electric field

Many previous studies^27,66–68 show that the band gaps of bilayer graphene and MoS₂ can be tuned by applying a vertical electric field. Therefore, it is interesting to investigate effect of the vertical external electric field (E-field) on the electronic properties of g-ZnO/MoS₂ hybrid structure. Only the AA, AB-Zn, and AB-O-stacked configurations with the lattice constant of isolated g-ZnO under the E-field are mainly discussed due to its higher stability than that of the constrained hybrid structure. Two directions of E-field with +E and −E perpendicular to g-ZnO and MoS₂ sheets are considered. The positive direction (+E) is defined as the direction from MoS₂ to g-ZnO, and the negative direction (−E) is its reverse direction.

Fig. 6 and S6 in ESI† show the geometric structures of g-ZnO/MoS₂ under the E-field with selected strength. It is found that the E-field has different effect on the geometric structures of supported g-ZnO, depending not only on the stacking pattern, but also on direction of field. For initial AB-Zn stacking, the configuration transformation from AB-Zn to AA for g-ZnO/MoS₂ can occur with increasing the E-field with +E direction (Fig. 6a and b), in particular, and the covalent bonds between Zn and its neighboring S atoms with distances of 2.43 Å are formed as the E-field strength is reached to 0.6 V Å⁻¹. Interestingly, the opposite E-field does not change the stacking order of hybrid structure but leads to the buckled structure of g-ZnO (Fig. 6c and d). For the initial AB-O stacking, the buckled structure of g-ZnO is also formed under the E-field with large strength, and this initial stacking order of g-ZnO/MoS₂ is unaffected (Fig. 6e–h).

Fig. 6 Top and side view of geometrical structures of g-ZnO/MoS₂ with initial (a–d) AB-Zn and (e–h) AB-O configurations under the E-field with different strength. The distance is in Å.

In the case of initial AA stacking, the basal plane structure and atomic arrangement of g-ZnO are retained for all the E-field with +E direction (see Fig. S6a and b†). Similarly, the positive E-field with larger strength obviously reduces the interlayer distance between g-ZnO and MoS₂, leading to the formation of covalent bond between Zn and its neighboring S atoms at last. However, when the E-field the −E direction is applied and continually increased, the buckled structure of g-ZnO is observed, and the AA stacking of g-ZnO/MoS₂ is transformed to the AB-O one (see Fig. S6c and d†).

The E-field greatly affects the stability of g-ZnO/MoS₂. The calculated results (Fig. 7a) show that the applied E-field improves the binding strength of g-ZnO/MoS₂, regardless of its direction. The binding energies for g-ZnO/MoS₂ increase with increasing the E-field. The enhanced interaction by applied E-field can also be supported by the reduced interlayer distance between g-ZnO and MoS₂ (see Fig. 7b), although this distances are slightly increased for AA stacking under the E-field with −E direction with small strength.

Fig. 7 (a) Binding energies, (b) interlayer distance, (c) band gap, (d) charge transfer from g-ZnO to MoS₂ for g-ZnO/MoS₂ with different initial stacking pattern as a function of the external electric field.

We now focus on the electronic properties of g-ZnO/MoS₂ under the E-field. The band gap of hybrid structure as a function of the field strength is plotted in Fig. 7c. Fig. 8 and S7† present the band structures of g-ZnO/MoS₂ under the E-field with selected strength. When the E-field with +E direction is applied, the g-ZnO/MoS₂ hybrid structures for all stacking patterns are still semiconductors, but its band gaps are obviously increased from 0.34 to 0.94 eV for AA, 0.71 to 1.11 eV for AB-O, and 0.62 to 1.07 eV for AB-Zn stacking with 0.1 V Å⁻¹. When the E-field continues to increase, the band gaps can further be increased, although those values are less sensitive to large E-field strength compared to 0.1 V Å⁻¹ (see Fig. 7c). It is noted that the small band gap of 0.51 eV for AB-O stacking with 0.6 V Å⁻¹ is attributed to the formation of buckled structure of g-ZnO (see Fig. 6b).

Fig. 8 Band structures of g-ZnO/MoS₂ with initial (a–d) AB-O and (e–h) AB-Zn stacking patterns under the external electric field with different strength. The Fermi level is set to 0.

Interestingly, in contrast to a large number of MoS₂-based layered nanocomposites,⁶⁸ when the E-field with −E direction is applied, the band structure calculations show that the semiconducting g-ZnO/MoS₂ becomes metallic one (Fig. 8d and h), regardless of the stacking pattern. Such semiconducting → metallic behavior transitions for g-ZnO/MoS₂ is related to the formation of buckled geometrical structure of g-ZnO supported on MoS₂ induced by the negative E-field. In addition, when the lattice constant of g-ZnO/MoS₂ is set to that of isolated MoS₂, the electronic property change from semiconducting to metallic behavior is still realized by the E-field (see Fig. S8†). The electric field-induced metal-semiconductor transition in g-ZnO/MoS₂ may provide an effective way to fabricate transistor semiconducting channels in further MoS₂-based devices. Such strategy has been reported in graphene-based transistors.^69,70

To further insight into the band structures of g-ZnO/MoS₂ under the E-field, we show the evolution of the band edges, E_CB and E_VB, as functions of the E-field as shown in Fig. 9, where E_CB and E_VB are the absolute energy level of the CBM and VBM of g-ZnO/MoS₂, respectively. The Fermi level of a semiconductor is not well defined, in fact, the branch-point energy⁷¹ may be chosen as a common energy reference between different systems. With increasing the positive E-field strength, usually, the energy level of CBM for AB-Zn (Fig. 9a) and AB-O (Fig. 9b) increases, whereas the values of VBM decrease for AB-Zn and slowly increase for AB-O, thus giving rise to increase of band gap. For the negative E-field, in contrast, the position of VBM is pushed up above the CBM, and the former is increased with increasing the E-field, while the latter is less changed. Therefore, the metallic property of g-ZnO/MoS₂ becomes more prominent as the negative E-field strength increases.

Fig. 9 The E_CB and E_VB of g-ZnO/MoS₂ with initial (a) AB-Zn and (b) AB-O stacking patterns as a function of the external electric field.

The modulation of band structure of g-ZnO/MoS₂ under the E-field can be understood by studying their electrostatic potential profile. Fig. 5b displays xy-averaged electrostatic potential of g-ZnO/MoS₂ with AB-Zn under different E-field. It is revealed that the E-field results in the distinct change in electrostatic potential in atom layer of g-ZnO and MoS₂ and especially in their interlayer regions close to g-ZnO, depending on its strength and direction. When the positive E-field is applied, the electrostatic potential in interlayer regions decreases with increasing the E-field (see insert in Fig. 5b), while these values in metal Mo layer slightly increase. Thus, the electrostatic potential difference between the atomic layer of MoS₂ and interlayer region under this E-field increases, which is responsible for increase of band gap. In contrast, with increasing the negative E-field, the electrostatic potential in interlayer region increases, but this potential in metal Mo layer slightly decreases, so the electrostatic potential difference between them decreases when the E-field changes from +E to −E direction, leading to metallic g-ZnO/MoS₂.

The remarkable effect of E-field on the electronic properties g-ZnO/MoS₂ is also illuminated by the charge transfer between g-ZnO and MoS₂ as shown in Fig. 7d. In the presence of +E, the interfacial interaction results in the p-type doping of MoS₂ by g-ZnO, in which the charge transfers from g-ZnO to MoS₂ become easier due to increased electrostatic potential difference between two sheets under this E-field. Thus, the induced hole carriers in MoS₂ increase with increasing the E-field. In contrary, the reverse electric field realizes the n-type doping of MoS₂ because electrostatic potential difference between MoS₂ and interlayer regions close to g-ZnO decreases with increasing the E-field.

The charge density difference of g-ZnO/MoS₂ with AB-O under E-filed further explains the larger charge transfers between these two component as shown in Fig. 4e–g. With increasing the E-field, the charge redistribution of g-ZnO/MoS₂ becomes more prominent, compared to the hybrid without E-field. The positive and negative E-field lead to opposite charge redistribution occurred on g-ZnO and MoS₂. The obvious difference of charge density in interlayer regions between positive and negative E-field is also clearly found (see Fig. 4e and f). For example, when the positive E-field is applied, electron accumulation mainly appears on g-ZnO, while MoS₂ loses electrons leading to electron depletion, which are enhanced with increasing the E-field. This results are consistent with the predicted charge transfer from ZnO to MoS₂ (see Fig. 7d) and with the changes in electrostatic potential of g-ZnO/MoS₂ under E-field (Fig. 5b). Therefore, the effective charge transfer between MoS₂ and g-ZnO indicates a novel way for MoS₂-based photoelectronic design.

4. Conclusions

In summary, we have investigated the structural and electronic properties of hybrid g-ZnO/MoS₂ nanocomposites via the first-principles calculations. Monolayer and bilayer MoS₂ are found to interact weakly with g-ZnO with the interlayer spacing of about 3.0 Å. The binding strength between MoS₂ and g-ZnO is sensitive to stacking pattern of g-ZnO/MoS₂.The interface interaction between g-ZnO and MoS₂ reduces the band gaps of g-ZnO/MoS₂, compared to these individual sheets, which depend on its stacking pattern. Interestingly, the electronic properties of g-ZnO/MoS₂ can be tuned by the E-field with direction normal to g-ZnO and MoS₂ sheets. The E-field with the positive direction from MoS₂ to g-ZnO increases the band gap of g-ZnO/MoS₂ nanocomposites, whereas these nanocomposites undergo the semiconductor to metal transitions when the E-field changes from positive to negative direction, regardless of its configuration. The changes in electrostatic potential induced by the E-field are responsible for modification of band structures of g-ZnO/MoS₂ nanocomposites. Our results offer a new way for fabricating high-performance MoS₂-based nanodevices.

Acknowledgements

This work was supported by the National Science Foundation of China (21103026, 21463004, and 21133007), the Educational Commission of Jiangxi Province (14653), and Jiangxi Provincial Natural Science Foundation (20151BAB203015). We acknowledge simulating discussions with Z. Cao and thank the computational resources and assistance provided by the State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University).

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Footnote

† Electronic supplementary information (ESI) available: Geometrical structures, band structures, partial charge densities, and DOS of g-ZnO/MoS₂ or bilayer MoS₂ with other configurations; band structures and DOS by HSE06, and geometrical and band structures of other hybrid structure under the E-field. See DOI: 10.1039/c5ra18114c

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