Electronic and optical properties of heterostructures based on transition metal dichalcogenides and graphene-like zinc oxide

The structural, electronic, and optical properties of heterostructures formed by transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se) and graphene-like zinc oxide (ZnO) were investigated using first-principles calculations. The interlayer interaction in all heterostructures was characterized by van der Waals forces. Type-II band alignment occurs at the MoS2/ZnO and WS2/ZnO interfaces, together with the large built-in electric field across the interface, suggesting effective photogenerated-charge separation. Meanwhile, type-I band alignment occurs at the MoSe2/ZnO and WSe2/ZnO interfaces. Moreover, all heterostructures exhibit excellent optical absorption in the visible and infrared regions, which is vital for optical applications.

are the total energy of MX 2 , ZnO, and the MX 2 / ZnO heterostructure, respectively. The binding energy of the MoS 2 /ZnO, WS 2 /ZnO, MoSe 2 /ZnO and WSe 2 /ZnO heterostructures are 269, 264, 285 and 282 meV respectively, while the corresponding interlayer distances are 2.91, 2.98, 2.89 and 2.89 Å respectively, indicating the typical vdW nature of the interaction between the two layers.
The projected band structures of the MoS 2 /ZnO, WS 2 /ZnO, MoSe 2 /ZnO, and WSe 2 /ZnO vdW heterostructures are shown in Fig. 2. These heterostructures can be divided into two categories. The first category includes the MoS 2 /ZnO and WS 2 /ZnO heterostructures, both of which have a type-II band alignment. They are semiconductors with indirect bandgaps of 1.60 and 2.05 eV, respectively. The CBM and VBM of the MoS 2 /ZnO (or WS 2 /ZnO) heterostructure are predominately contributed by the MoS 2 (or WS 2 ) and ZnO layers respectively. The second category includes the MoSe 2 /ZnO and WSe 2 /ZnO heterostructures, both of which have a type-I band alignment. Both the CBM and VBM of MoSe 2 /ZnO and WSe 2 /ZnO heterostructures are located at the K point in BZ, which suggest that they are direct-bandgap semiconductors. The bandgaps of MoSe 2 /ZnO and WSe 2 /ZnO heterostructures are 1.96 and 2.08 eV, respectively. Moreover, both the CBM and VBM of these two heterostructures originate from the TMD layer.
Previously, many reports [41][42][43] suggested that MoS 2 and WS 2 have the potential for application in photocatalysts for water splitting. The main obstacle to obtain a high-efficiency photocatalyst is the problem of electronhole recombination. In the MoS 2 /ZnO and WS 2 /ZnO heterostructures, the conduction-band offset (CBO) and valence-band offset (VBO) between the MoS 2 (or WS 2 ) and ZnO layers are approximately 2.49 (or 2.00) and 0.58 (or 0.26) eV respectively, as shown in Fig. 3(a). Driven by the CBO, the photogenerated electrons in ZnO tend to move to the CB of the MoS 2 (or WS 2 ) layer, while the photogenerated holes in the MoS 2 (or WS 2 ) layer are readily migrate to the VB of the ZnO layer with the assistance of the VBO. Therefore, the problem of electron-hole recombination can be overcome with these band offsets.
The built-in electric field plays an important role in determining the catalytic activity of a photocatalyst because a large built-in electric field can further boost the migration of photogenerated charges. The insets in Fig. 4 present the isosurfaces of charge difference of the MoS 2 /ZnO and WS 2 /ZnO vdW heterostructures. The ZnO layer always acts as a donor. The transferred charge is 0.016 (or 0.012) |e| for the MoS 2 /ZnO (or WS 2 /ZnO) vdW heterostructure according to the Bader charge-population analysis [44][45][46] , which can induce a large potential drop across the interface of the heterostructure, as shown in Fig. 4. The potential drop across the MoS 2 /ZnO (or WS 2 /ZnO interface) is 7.38 (or 7.33) eV, which can induce a large built-in electric field from the MoS 2 (or WS 2 ) layer to the ZnO layer, and this field should exert some effect on the photogenerated electron-hole recombination in the MoS 2 /ZnO (or WS 2 /ZnO) vdW heterostructure. The CBO and VBO in the MoSe 2 /ZnO and WSe 2 /ZnO heterostructures also play an important role. As shown in Fig. 3(b), the CBO and VBO in the MoSe 2 /ZnO (or WSe 2 /ZnO) heterostructure are 1.37 (or 1.07) and 0.53 (or 0.78) eV, respectively. With these band offsets, both the photogenerated electrons and holes tend to move from the ZnO to the TMD layer, while the photogenerated electrons and holes in the TMD layer are prohibited from escaping. Thus, the photogenerated electrons and holes tend to recombine again, which would be a useful feature for optical devices such as LEDs 30 .
The imaginary parts of the dielectric functions of the MoS 2 /ZnO, WS 2 /ZnO, MoSe 2 /ZnO, and WSe 2 /ZnO vdW heterostructures are shown in Fig. 5. All the heterostructures show good ability to absorb light in the visible  and near-infrared (NIR) regions, which is evident from the high absorption peaks at approximately 488, 555, 441, and 498 nm in the visible region of their respective spectra. Since the wavelengths of light arriving at the earth are mainly in the visible and NIRregions 47 , these heterostructures are promising components for various optical, photovoltaic and photocatalytic applications.
In summary, the structural, electronic, and optical properties of the MoS 2 /ZnO, WS 2 /ZnO, MoSe 2 /ZnO, and WSe 2 /ZnO vdW heterostructures were systematically investigated using first-principles calculations. The interactions at all the TMD/ZnO interfaces are dominated by vdW forces. The MoS 2 /ZnO and WS 2 /ZnO vdW heterostructures are indirect-bandgap semiconductors with bandgaps of 1.60 and 2.05 eV, respectively. The CBM is contributed by the TMD layer, while the VBM is contributed by the ZnO layer, indicating the formation of a type-II heterostructure, which can promote the separation of photogenerated electron-hole pairs. Moreover, large built-in electric fields are stabilized at both the MoS 2 /ZnO and WS 2 /ZnO interfaces, which will further separate the photogenerated charges. On the other hand, the MoSe 2 /ZnO and WSe 2 /ZnO vdW heterostructures are direct-bandgap semiconductors with bandgaps of 1.96 and 2.08 eV respectively. Both the CBM and VBM originate from the TMD layer, thus a type-I heterostructure is formed. In addition, the MoS 2 /ZnO, WS 2 /ZnO, MoSe 2 / ZnO, and WSe 2 /ZnO vdW heterostructures are all high solar-flux collectors. Therefore, these hetersotructures have great potential for application in optical, photovoltaic, and photocatalytic devices.

Methods
First-principles calculations were carried out by using the Vienna Ab Initio Simulation Package 48 , which is based on the density functional theory (DFT) in a plane-wave basis set with the projector-augmented wave method 49 . For the exchange-correlation functional, the generalized gradient approximation of Perdew, Burke, and Ernzerhof 50,51 was used to obtain the geometric structures, while the Heyd-Scuseria-Ernzerhof hybrid functional 52,53 was used to calculate the electronic and optical properties. The DFT-D3 method of Grimme 54 was used to account for the dispersion forces. The energy cutoff for plane-wave expansion was set to 550 eV, and the first Brillouin zone was sampled by a 21 × 21 × 1 Monkhorst-Pack 55 k-point grid. The thickness of the vacuum region