Electric field tunability of the electronic properties and contact types in the MoS2/SiH heterostructure

The generation of layered heterostructures with type-II band alignment is considered to be an effective tool for the design and fabrication of a highly efficient photocatalyst. In this work, we design a novel type-II MoS2/SiH HTS and investigate its atomic structure, electronic properties and contact types. In the ground state, the MoS2/SiH HTS is proved to be structurally and mechanically stable. The MoS2/SiH HTS generates type-II band alignment with separation of the photogenerated carriers. Both the electronic properties and contact type of the MoS2/SiH HTS can be modulated by an external electric field. The application of a negative electric field leads to a transformation from type-II to type-I band alignment. While the application of a positive electric field gives rise to a transition from semiconductor to metal in the MoS2/SiH HTS. These results could provide useful information for the design and fabrication of photoelectric devices on the MoS2/SiH HTS.


Introduction
The successful isolation of graphene 1 has opened up a new research direction in novel materials science. Over nearly one decade, a series of two-dimensional materials [2][3][4] have been discovered, synthesized experimentally and investigated systematically. 2D materials are proved to have outstanding physical and chemical properties that make them promising candidates for various applications, including transistors, 5 Liion batteries, 6 water splitting 7 and gas sensors. 8 Graphene, 1 hexagonal boron nitride (h-BN), 9 transition metal dichalcogenides (TMDs) 10 and phosphorene analogues 11,12 are currently considered to be the most attractive 2D materials owing to their intriguing properties and wide range of applications. Although these 2D materials have promising properties, they also exhibit some disadvantages that may limit their applications in various elds. For instance, the lack of band gap in graphene 13 limits its application in eld-effect transistors. 14 Unlike graphene, phosphorene is a semiconductor with a nite band gap. However, the structural instability under ambient conditions of phosphorene limits its application in modern day devices. 15 Therefore, the search for novel 2D materials as well as nding common approaches to control their properties for various applications is still challenging.
Currently, one of the most commonly used techniques to improve the properties and expand the application range of 2D materials is the generation of van der Waals (vdW) heterostructures (HTSs) between two or more different 2D materials. 16,17 The different 2D materials in their vdW-HTSs are held together by the weak vdW forces. Thus, the intrinsic excellent properties of the constituent 2D materials are maintained in their combined HTSs. Moreover, the formation of vdW-HTSs between 2D materials may also give rise to the creation of novel properties that may not exist in the constituent 2D monolayers. Generally, the recombination of the photogenerated carriers in single-layer 2D materials is very rapid. Whereas, the photogenerated carriers in the combined vdW-HTSs between different 2D materials are separated effectively depending on the positions of the band edges of the constituent monolayers. The combination between two different 2D materials results in the formation of type-I, type-II or type-III band alignment, as depicted in Fig. S1 of the ESI. † For type-I, the conduction band minimum (CBM)/valence band maximum (VBM) of one layer is higher/lower than that of the other layer. In type-II, the CBM and VBM of the heterostructure come from different constituent monolayers. In type-III, the CBM of one layer is lower than the VBM of the other layer. To date, a large number of HTSs have been formed between two or more different 2D materials, such as graphene HTSs 18-21 and TMD HTSs. [22][23][24][25] The combination between two or more different 2D materials may also give rise to the appearance of novel properties that may not be observed in the 2D monolayers.
Recently, a novel 2D material, namely silane (SiH), was obtained by the covalent modication of hydrogen and silicene. 26,27 Unlike silicene, the SiH monolayer is a semiconductor with a band gap of about 2.19 eV, 28 making it suitable for photocatalysis and optoelectronic applications. 29,30 The SiH monolayer is structurally stable at room temperature. The combination between the SiH monolayer and other 2D materials has been proposed and predicted recently. For instance, Han et al. investigated type-II band alignment in the GaAs/SiH HTS using rst principles calculations. 31 The results showed that the formation of the type-II GaAs/SiH HTS leads to an enhancement of the optical absorption in the visible light region. Zeng et al. 29 constructed a novel SiH/CeO 2 (111) HTS and investigated its electronic and optical properties and photocatalytic performance. The results demonstrated that the novel SiH/CeO 2 (111) HTS generates type-II band alignment and it is a promising photocatalyst for splitting water to hydrogen. All the above-mentioned ndings suggest that the SiH monolayer can be used to form HTSs with other 2D materials. Currently, MoS 2 is one of the most attractive materials in the 2D TMD family. 32 HTSs between MoS 2 and other 2D materials have not only been predicted theoretically 33,34 but also synthesized experimentally. [35][36][37] However, to date, the combination between single layer SiH and MoS 2 monolayers has not yet been designed and investigated.
In this work, we perform rst principles calculations to design a novel MoS 2 /SiH HTS and investigate its atomic structure, electronic properties and contact types. Our results show that the MoS 2 /SiH HTS is structurally and mechanically stable in the ground state. The MoS 2 /SiH HTS generates type-II band alignment, making it a promising candidate as an efficient photovoltaic device because the photogenerated carriers in type-II are separated in the two materials. Both the electronic properties and contact type of the MoS 2 /SiH HTS can be modulated by an external electric eld. The application of a negative electric eld leads to a transformation from type-II to type-I band alignment. While the application of a positive electric eld gives rise to a transition from semiconductor to metal in the MoS 2 /SiH HTS. These results could provide useful information for the design and synthesis of photocatalytic devices based on the MoS 2 /SiH HTS.

Computational methods
All calculations are performed in the framework of density functional theory (DFT) using rst-principles calculations. 38 The Vienna ab initio simulation (VASP) 39,40 is used to perform the structural optimization and electronic properties prediction. The Perdew-Burke-Ernzerhof (PBE) functional in the framework of the generalized gradient approximation (GGA) 41,42 is used to describe the exchange-correlation force. The HSE06 functional is chosen to obtain more accurately the band gaps of the materials. 43 A vacuum space of 20Å is used to avoid all unnecessary neighboring layered interactions in materials. A k-point mesh of 12 Â 12 Â 1 and a cut-off energy of 510 eV are chosen. All structural geometries are fully optimized until the convergence tolerance of energy and force which are less than 10 À6 eV and 10 À3 eVÅ À1 , respectively. A DFT-D3 method 44 is also adopted to describe the long-range forces in the layered 2D materials.

Results and discussion
We rst investigate the atomic structure and electronic properties of MoS 2 and SiH monolayers. The atomic structures of these monolayers are depicted in Fig. 1(a) and (e). The lattice constants of MoS 2 and SiH monolayers aer the geometric optimization are calculated to be 3.18 and 3.86Å, respectively. These values are in good agreement with the previous reports. 26,29,[45][46][47] The band structures of MoS 2 and SiH monolayers obtained by the PBE and HSE functionals are depicted in Fig. 1 Both the PBE and HSE functionals predict the same characteristics of MoS 2 and SiH monolayers, suggesting that the PBE method predicts the correct trends and physical mechanisms for the MoS 2 /SiH heterostructure. Thus, we decide to choose the PBE functional for all the next calculations because of a low computational cost. Moreover, our goal is not to acquire the precise band gaps of the MoS 2 /SiH heterostructure, but to explore the trend in electronic properties and contact types under external factors, such as external electric eld.
We next design the combination between two different MoS 2 and SiH monolayers to form the MoS 2 /SiH heterostructure. We designed ve different stacking congurations of the MoS 2 /SiH heterostructure. The most energetically stable stacking conguration of the MoS 2 /SiH heterostructure is shown in Fig. 2, while the other stacking congurations are illustrated in Fig. S2 of the ESI. † It should be noted that the stacking conguration of the MoS 2 /SiH heterostructure presented in the manuscript is the most energetically stable heterostructure because it has the lowest binding energy compared to the others. In order to design the MoS 2 /SiH heterostructure, we use a supercell, consisting of (2 Â 2) unit cells of the MoS 2 layer and ð ffiffiffi 3 p Â ffiffiffi 3 p Þ unit cells of the SiH monolayer. The lattice constant of the MoS 2 /SiH heterostructure aer geometric optimization is found to be 6.43Å. The lattice mismatch in the MoS 2 /SiH heterostructure is calculated to be less than 2%, which is small and affects insignicantly the electronic features of the heterostructure. Aer geometric optimization, the interlayer spacing between MoS 2 and SiH layers in the corresponding heterostructure is 2.48Å. This interlayer spacing is in the same order of magnitude as other Si(Ge)H-based heterostructures, such as GeH/graphene, 48 SiH/PtSe 2 , 47 and SiH/AlAs. 49 Moreover, to check the structural stability of such a heterostructure, we further calculate the binding energy as:  Fig. 3(a). The contributions of MoS 2 and SiH layers are represented by blue and red circles, respectively. One can nd that the MoS 2 /SiH HTS possesses a semiconducting nature with an indirect band gap of 0.26 eV, which is smaller than that of both the MoS 2 and SiH monolayers. This nding indicates that the generation of the MoS 2 /SiH HTS gives rise to a reduction in the band gap compared to that of the constituent monolayers. More interestingly, the VBM of the MoS 2 /SiH HTS is located at the G point and it comes mainly from the SiH layer. Whereas, the CBM of the MoS 2 /SiH HTS is located at the M point and it is mainly contributed by the MoS 2 layer. These contributions demonstrate that the VBM and CBM of the MoS 2 /SiH HTS come from different layers, resulting in the formation of type-II band alignment. The type-II band alignment of the MoS 2 /SiH HTS makes it a promising candidate as an efficient photovoltaic device because the photogenerated carriers in type-II are separated in the two materials.
The generation of type-II band alignment in the MoS 2 /SiH HTS may lead to charge redistribution at the interface. Therefore, we further visualize the charge density difference as Dr ¼ r H À r M À r S , where r H , r M and r S are the charge densities of the MoS 2 /SiH HTS, and isolated MoS 2 and SiH monolayers, respectively. One can observe from Fig. 3(b) that there is a charge redistribution at the interface of the MoS 2 /SiH HTS. In addition, we nd that the SiH layer loses electrons, while the MoS 2 layer gains electrons. This indicates that the charges are transferred from SiH to MoS 2 layers in the MoS 2 /SiH HTS.  Furthermore, the electrostatic potential of the MoS 2 /SiH HTS in Fig. 3(c) demonstrates that the MoS 2 layer has a deeper potential than the SiH layer, causing the creation of a built-in electric eld. The potential drop between the MoS 2 and SiH layers is 4.2 eV. It is obvious that the formation of a built-in electric eld at the interface of the MoS 2 /SiH HTS gives rise to the separation of the photogenerated electrons and holes as well as the inhibition of their recombination.
Furthermore, to have a better understanding of the charge transfer between two different MoS 2 and SiH layers, we calculate the work function of the MoS 2 /SiH HTS as well as the constituent MoS 2 and SiH monolayers for comparison. The work function can be calculated as: F ¼ E vac À E F , where E vac and E F are the vacuum energy and the Fermi level energy, respectively. The calculated work functions of the MoS 2 /SiH HTS, and isolated MoS 2 and SiH monolayers are 4.62, 5.15 and 5.80 eV, respectively. The difference in the work functions of the MoS 2 and SiH monolayers allows the charge transfer from the SiH to the MoS 2 layer. Furthermore, the work function of the MoS 2 /SiH HTS is lower than that of both the MoS 2 and SiH monolayers, suggesting that there is a charge redistribution at the interface of the heterostructure.
Currently, applying an electric eld (E) is proven to be an effective tool to modify the electronic properties and contact types in the HTS between two different 2D materials. The strength of the applied E ranges from À0.4 VÅ À1 to +0.4 VÅ À1 . The direction of the applied E is from the SiH to the MoS 2 layer, as depicted in Fig. 4(a). It should be noted that the electric eld was applied for both the optimization process and electronic properties calculations. Our results showed that the relaxed atomic structures of the MoS 2 /SiH heterostructure remain unchanged when the electric eld is applied in the optimization process as compared to those without the application of the electric eld, as depicted in Fig. S3 of the ESI. † The dependence of the band gap of the MoS 2 /SiH HTS as a function of applied E is depicted in Fig. 4(b). One can nd that a positive E leads to a narrower band gap of the MoS 2 /SiH HTS, while a negative E gives rise to an increase in the band gap of the MoS 2 /SiH HTS. Under the positive E ¼ +0.4 VÅ À1 , the band gap of the MoS 2 /SiH HTS is reduced to zero, indicating that the semiconducting nature in the MoS 2 /SiH HTS transforms into metallic nature. Moreover, it is obvious that the band gap changes of the   SiH layers in the corresponding MoS 2 /SiH HTS, we predict that the VBM of the SiH layer will be lower than that of MoS 2 at the critical strength of the negative E ¼ À0.7 VÅ À1 . This prediction suggests that a transformation from type-II to type-I band alignment will be achieved under the application of a negative electric eld of À0.7 VÅ À1 . However, it would not be reasonable to apply higher E values, since they would be rather difficult to experimentally achieve. In addition, it should be noted that to apply an electric eld in devices, experimental realization is considered as one pole of the battery connecting to MoS 2 , and the other pole connecting to the SiH layer. Thus, we predict that the MoS 2 /SiH heterostructure is placed inside the capacitor conguration, from which the electric eld is generated. The electric eld penetrates through the whole MoS 2 layer. The electric eld outside and inside the MoS 2 /SiH heterostructure is the same when the out-of-plane dielectric polarization is calculated to be zero.

Conclusions
In conclusion, we have designed a type-II MoS 2 /SiH HTS and investigated its structural and electronic properties and the formation of the contact types by performing rst principles calculations. The MoS 2 /SiH HTS is proved to be structurally and mechanically stable in the equilibrium state. The formation of the MoS 2 /SiH HTS leads to a reduction of the band gap compared to that of the constituent MoS 2 and SiH monolayers, indicating that the electrons move quickly from the VB to the CB in such a HTS. Moreover, the MoS 2 /SiH HTS forms type-II band alignment, indicating that such a HTS is a promising candidate as an efficient photoelectric device because the photogenerated carriers in type-II are separated in each material. Furthermore, the electronic properties and contact type of the MoS 2 /SiH HTS can be modulated by an external electric eld. The application of a negative electric eld leads to a transformation from type-II to type-I band alignment. While the application of a positive electric eld gives rise to a transition from semiconductor to metal in the MoS 2 /SiH HTS. These results could provide useful information for the design and synthesis of photoelectric devices based on the MoS 2 /SiH HTS.

Conflicts of interest
There are no conicts to declare.