Tunable Band Alignment in the Arsenene/WS₂ Heterostructure by Applying Electric Field and Strain

Abstract

Arsenene has received considerable attention because of its unique optoelectronic and nanoelectronic properties. Nevertheless, the research on van der Waals (vdW) heterojunctions based on arsenene has just begun, which hinders the application of arsenene in the optoelectronic and nanoelectronic fields. Here, we systemically predict the stability and electronic structures of the arsenene/WS₂ vdW heterojunction based on first-principles calculations, considering the stacking pattern, electric field, and strain effects. We found that the arsenene/WS₂ heterostructure possesses a type-II band alignment. Moreover, the electric field can effectively tune both the band gap and the band alignment type. Additionally, the band gap could be tuned effectively by strain, while the band alignment type is robust under strain. Our study opens up a new avenue for the application of ultrathin arsenene-based vdW heterostructures in future nano- and optoelectronics applications. Our study demonstrates that the arsenene/WS₂ heterostructure offers a candidate material for optoelectronic and nanoelectronic devices.

1. Introduction

Atomically thin two-dimensional (2D) materials have received continuous attention due to their novel physical and chemical properties [1,2,3,4,5]. Furthermore, a large number of 2D materials, such as phosphorene and stanine, have been successfully applied to field-effect transistors and photodetectors [6,7,8,9]. Recently, as with the analogs of 2D phosphorene, arsenene has gained a surge in research interest [10,11,12,13,14]. Arsenene possesses a rhombohedral monolayer structure, which is isolated from gray arsenic. High carrier mobility and excellent visible light absorption capacity make arsenene a candidate for photovoltaic and photocatalytic materials [15,16,17]. Studies have demonstrated that strain could effectively modulate not only the type of band gap but also the effective mass of holes and electrons in arsenene [18,19]. Moreover, Song et al. studied the effect of defects on the transport properties of arsenene; they found that the current was tremendously enhanced [20]. In addition, arsenene has potential applications in magneto-optical devices. The spin polarization along the out-of-plane direction promotes the magneto-optical effect wherever the spin polarization in-plane weakens the magneto-optical effect. The magneto-optical Kerr angle of arsenene is larger than that of other materials, due to the stronger spin-orbit coupling [21]. The above studies show that arsenene is an attractive material in the optoelectronic and nanoelectronic fields.

Nonetheless, individual 2D materials with restricted properties cannot meet the requirements of nanoelectronic and optoelectronic devices. For instance, graphene is gapless, but a band gap is crucial in many graphene-based electronic devices [22,23]. Therefore, in order to meet the desired properties for nanoelectronics and optoelectronic devices, different methods have been proposed to tune the characters of individual 2D materials [24,25,26,27,28]. Among these strategies, vdW heterostructures were constructed via stacking vertically various monolayer 2D materials; the desired functionality was realized by means of selecting appropriate materials and controlling vdW interaction at the interface. Therefore, fabricating a 2D materials-based vdW heterostructure has been widely considered to be a very effective approach to breaking through the limitations of individual 2D materials and gaining novel electronic and optoelectronic features.

In this contribution, we studied the arsenene/WS₂ heterojunction. The reason for using WS₂ as the building blocks of vdW heterostructure is given as follows. Firstly, both arsenene and WS₂ possess a hexagonal lattice similar to graphene. Secondly, monolayer WS₂ possesses a direct band gap and redshifted charged excitons. In recent years, 2D ultrathin hybrid WS₂-based vdW heterostructures have been synthesized experimentally [29,30,31]. Moreover, WS₂-based vdW heterostructures indicate novel properties that cannot be found in individual 2D materials. For example, Lan et. al. [32] reported that the photocarrier transport of monolayer WS₂ was tremendously promoted by stacking the ZnO/WS₂ heterostructure. Thus, the first question arises: can arsenene and WS₂ form an arsenene/WS₂ heterostructure, and can some new properties beyond individual arsenene and WS₂ be expected? For the semiconducting vdW heterostructures, there are three band alignment types, namely, typeⅠ, typeⅡ, and type Ⅲ. TypeⅠ, typeⅡ, and type Ⅲ band alignments have been applied to photodetectors [33,34], photovoltaic devices [35,36,37], and tunneling field effect transistors [38], respectively. Nevertheless, it is necessary to tune the band alignment type for manufacturing multi-functional devices. Naturally, the second question arises: can the band alignment of the arsenene/WS₂ heterostructure be tuned via different strategies? To address the above questions, in this contribution, we systematically studied the electronic properties of the arsenene/WS₂ heterostructure, considering the stacking configuration, electric field, and strain effects.

2. Computing Method

Our calculations were performed based on the density functional theory (DFT), as implemented in the Vienna ab initio simulation package (VASP) [39]. The electron–ion interaction was addressed using the projector augmented wave method (PAW) [40]. We used the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhofer parameterization (PBE) for describing the exchange-correlation potential [41]. The weak vdW interaction was described using the DFT-D2 method [41,42]. The energy cutoff of the plane wave was selected to 500 eV. The Brillouin zone integration was performed by means of a 15 × 15 × 1 grid for all calculations. The maximum Hellmann–Feynman force and total energy convergence criterion were set to 0.01 eV/Å and 10⁻⁵ eV, respectively. The thickness of the vacuum space was set at 25 Å to avoid any interaction between adjacent layers.

3. Results and Discussion

3.1. Geometric Structure of the Arsenene/WS₂ Heterostructure

To theoretically model the heterostructure, we constructed the arsenene/WS₂ heterostructure by stacking a $\sqrt{3}\times \sqrt{3}$ arsenene supercell on top of a 2 × 2 WS₂ supercell. The lattice mismatch value is less than 2.5%. Meanwhile, in order to investigate the stacking effect on the arsenene/WS₂ heterostructure, we consider three representative stacking configurations, namely, the bottom, top, and center configurations, which is displayed in Figure 1. Furthermore, to evaluate the structural stability of the arsenene/WS₂ heterostructure, we calculated the binding energy (E_b), which can be described as follows:

$$E_b=E_{As/W}-E_{As}-E_W$$

where E_As/W, E_As, and E_W are the total energies of the arsenene/WS₂ heterostructure, arsenene, and monolayer WS₂, respectively.

Table 1. The interlayer distance, D, and binding energy, E_b, of the arsenene/WS₂ heterostructure for three stacking configurations.

Stacking	Bottom	Top	Center
D (Å)	3.31	3.28	3.32
Eb (meV)	−746.13	−751.20	−743.93

shows that the binding energies of the bottom, top, and center configurations are −746.13 meV, −751.20 meV, and −743.93 meV, respectively, which implies that all the stacking configurations are energetically stable. Based on the binding energy listed in Table 1, the trend of stability is top > bottom > center, indicating that top stacking is the most stable stacking configuration. Meanwhile, top stacking has the shortest interlayer distance of 3.28 Å. The shorter interlayer distance demonstrates a stronger vdW interaction between arsenene and WS₂, which may bring about new electronic properties in the arsenene/WS₂ heterostructure. Consequently, we will only discuss top stacking below.

3.2. Electronic Properties of the Arsenene/WS₂ Heterostructure

Here, we calculated the band structure of arsenene, monolayer WS₂, and the arsenene/WS₂ heterostructure, which is plotted in Figure 2. For arsenene, as depicted in Figure 2c, the conduction band minimum (CBM) lies at the Γ point and the valence band maximum (VBM) lies at the area between the M and Γ points, indicating an indirect band structure with a gap value of 1.32 eV. In the monolayer WS₂, both the VBM and CBM lie at the K point; thus, a direct gap value of 1.82 eV was obtained. After the arsenene and monolayer WS₂ have been combined, Figure 2a shows the band structure of the arsenene/WS₂ heterostructure. In Figure 2a, the blue and red lines represent the bands of arsenene and WS₂, respectively. Compared with Figure 2b,c, the band shapes of arsenene and WS₂ in the heterobilayer have hardly changed, which is the simple sum of the WS₂ and arsenene bands. Moreover, the CBM that is located at point K arises from WS₂ and the VBM, located at the point between the Γ and M points, and originates from arsenene. Therefore, the arsenene/WS₂ heterostructure possesses a type-II band alignment characteristic, which can spontaneously separate the electrons and holes in the heterostructure.

Notably, band alignment plays a key role in the performance of nanoelectronic devices; thus, we have plotted the band alignment of monolayers and the arsenene/WS₂ heterostructure in Figure 3. It can be seen that the work functions of arsenene, monolayer WS₂, and the arsenene/WS₂ heterostructure are 5.55 eV, 5.58 eV, and 5.17 eV, respectively. When arsenene and monolayer WS₂ are combined, electrons transfer from arsenene to the WS₂ layer because of the larger work function of WS₂. Thus, the Fermi level of arsenene (WS₂) moves downward (upward) and finally comes up to the same level. Consequently, arsenene accumulates a positive charge while WS₂ gathers a negative charge, which brings about an intrinsic electric field in the heterostructure. Such an intrinsic electric field can confine holes and electrons to the arsenene layer and WS₂ layer, respectively. This spontaneous separation delays the recombination of electrons and holes, which prolongs the lifetime of carriers in the heterostructure.

3.3. Influence of the Electric Field on Electronic Properties

It is very important to study the tunable electronic properties of the arsenene/WS₂ heterojunction, due to the remarkable effect of material properties on the performance of devices. Here, we explore the band structure of the arsenene/WS₂ heterojunction under various electric fields, as is demonstrated in Figure 4. The positive electric field is taken as that from WS₂ to arsenene. As can be seen in Figure 4, the larger the electric field is, the smaller the band gap; eventually, the band gap reaches zero. At 0 V/nm, the arsenene/WS₂ heterojunction is an indirect band gap and type-II semiconductor. When the electric field is applied to the arsenene/WS₂ heterostructure, the CBM is always at point K, while the VBM moves to the Γ point, which indicates that the arsenene/WS₂ heterostructure remains an indirect band-gap characteristic. It is clear that the electric field that drives the CBM shifts downward quickly, relative to the Fermi level; however, the VBM moves upward slowly, relative to the Fermi level. Ultimately, the band gap is closed at 2.25 V/nm.

To further explore the mechanism of the electric field effect on the band alignment of the arsenene/WS₂ heterostructure, we present the variations in band edge under an electric field in Figure 5. Figure 5 demonstrates that the CBM and VBM of arsenene increase continuously under a positive electric field, whereas the CBM and VBM of WS₂ decrease continuously. A positive electric field promotes the electron transfer from arsenene to WS₂, moving the bands of arsenene upward, and that of WS₂ downward. It should be noted that the CBM of WS₂ is always higher than the VBM of arsenene under a positive electric field, which demonstrates that the arsenene/WS₂ heterostructure remains in a type-II band alignment. Conversely to a positive electric field, a negative electric field drives the electron transfer from WS₂ to arsenene, shifting the bands of WS₂ upward and that of arsenene downward. Once the negative electric field is larger than −0.38 V/nm, this stronger negative electric field makes the CBM of arsenene shift down to be located below that of WS₂. Meanwhile, the VBM of arsenene lies above that of WS₂, where both the CBM and VBM of the heterostructure are contributed by arsenene. Therefore, the arsenene/WS₂ heterostructure transforms from a type-II band alignment into a type-I band alignment at −0.38 V/nm. With the further enhancement of the negative electric field, the VBM of WS₂ moves upward and is located above that of arsenene, whereas the CBM of the heterostructure can be attributed to the arsenene layer; type-II band alignment is constructed again at −0.95 V/nm. Thus, our work demonstrates that an electric field can effectively tune the band alignment types of an arsenene/WS₂ heterostructure, which offers a building block for fabricating multi-functional nanoelectronic devices.

3.4. The Effect of Strain on Electronic Properties

It is necessary to study the strain effect on the electronic structures of the arsenene/WS₂ heterojunction, owing to the widespread strain seen in experiments. Firstly, we apply the out-of-plane strain to the arsenene/WS₂ heterojunction. Strain can be described as $\epsilon =\left(d-d_0\right)/d_0$, where d and d₀ are the strained and unstrained interlayer distances, respectively. As can be seen from Figure 6a, the band gap decreases steadily; eventually, gap-closing occurs at −33% strain. This is because the smaller interlayer distance results in a stronger interaction in the heterostructure, which leads to more transferred charge. The Bader charge analysis shows that 0.048, 0.097, 0.1606, and 0.2962 electrons flow from arsenene to WS₂, with strains of 0%, −10%, −20%, and −30%, respectively. Therefore, greater charge transfer can bring about a stronger internal electric field, which decreases the band gap of the heterostructure. Conversely, positive strain drives the band gap increases gradually. This can be attributed to the fact that the interaction became weak and transferred electrons declined with the increasing interlayer distance. The Bader charge analysis demonstrates that 0.048, 0.031, 0.024, and 0.015 electrons transfer from arsenene to WS₂, with strains of 0%, 10%, 20%, and 30%, respectively. Less charge transfer means less of a wave function overlap of adjacent single layers, which narrows the energy band and then increases the band gap.

Figure 6b gives the band edge of the arsenene/WS₂ heterostructure under various out-of-plane strains. It is clear that the CBM of WS₂, the VBM of WS₂, and the CBM of arsenene shift upward quickly with positive strain; however, the VBM of arsenene moves upward slowly. Conversely, when negative strain is applied to the heterostructure, the CBM of WS₂, the VBM of WS₂, and the CBM of arsenene move down quickly, whereas the VBM of arsenene shifts upward slightly. Whether a positive strain or negative strain is applied, the CBM and VBM of arsenene are always higher than that of WS₂, and the type-II; band alignment of the arsenene/WS₂ heterostructure is robust under out-of-plane strain.

Finally, we would like to point out that a strain above 15% could barely be achieved, while it is acceptable to present an interlayer distance with a strain of 20–30%. Related studies [43,44,45] demonstrate that the strain based on interlayer distance can exceed 30%. Moreover, it should be noticed that the interlayer distance of the WS₂/MoS₂ heterostructure could be modulated by vacuum thermal annealing [46]. Therefore, we can expect that the interlayer distance of the vdW heterostructures can be tuned by certain experimental methods in the future.

4. Conclusions

In conclusion, we have explored the electronic properties of the arsenene/WS₂ heterostructure under electric fields and strains by means of first-principles calculations. The results demonstrate that the most stable stacking order is the top conformation of the three configurations. The negative binding energy indicates the thermodynamic stability of the arsenene/WS₂ heterostructure. The band gap is effectively tuned by the electric field and, ultimately, the gap is closed. In particular, the electric field can modulate the band edges of the arsenene/WS₂ heterostructure and can result in a transition between type-II and type-I band alignment. The band gap can be controlled by the strain, while type-II; band alignment is robust under strain. Our work indicates that the arsenene/WS₂ heterojunction is a building block for fabricating optoelectronic and nanoelectronic devices.