Effects of Vacancy Defects on Electrical and Optical Properties of ZnO/WSe₂ Heterostructure: First-Principles Study

Abstract

In this work, based on the first principles calculation of density functional theory (DFT), we studied the band structure changes of monolayer ZnO and ZnO/WSe₂ before and after vacancy generation, and systematically studied the vacancy formation energy, band structure, density of states, electronic density difference and optical properties of ZnO/WSe₂ heterostructure before and after vacancy generation. The results show that the band structures of ZnO, WSe_2, and ZnO/WSe₂ heterostructure are changed after the formation of Zn, O, W, and Se vacancies. The bandgap of the ZnO/WSe₂ heterostructure can be effectively controlled, the transition from direct to indirect bandgap semiconductor will occur, and the heterostructure will show metallic properties. The optical properties of heterostructure have also changed significantly, and the absorption capacity of heterostructure to infrared light has been greatly increased with red shift and blue shift respectively. The generation of vacancy changes the electrical and optical properties of ZnO/WSe₂ heterostructure, which provides a feasible strategy for adjusting the photoelectric properties of two-dimensional optoelectronic nano devices and has good potential and broad application prospects.

1. Introduction

In recent years, the shortage of fossil fuels and environmental degradation have forced the search for a clean, renewable fuel, and the conversion of visible light into hydrogen fuel is an advanced technology with great potential for application [1,2]. In 1972, Fujishima and Honda first used TiO₂ for water separation [3]. Two-dimensional nano-materials such as germanium, graphene, and transition metal-sulphur compounds, are widely used in the study of optoelectronic materials [4,5] and photocatalytic materials [6,7,8] because of their unique physical properties. Semiconductors are used in a wide range of applications, such as various electronic devices, photodetectors devices [9] and photovoltaic cells [10,11,12].

In recent decades, researchers have extensively explored the properties of semiconductors. Semiconductors doped with magnetic impurities, which are referred to as dilute magnetic semiconductors, have been developed toward spintronic applications [13,14]. These technologies usually rely on heterostructures fabricated under precisely controlled conditions [15]. Lattice defects have an important effect on the performance of devices and even play a decisive role [16,17,18]. Therefore, lattice defects play an important role in material properties or device properties [19]. For example, Qin et al. created vacancy defects in the ZnIn₂S₄/g-C₃N₄ heterojunction by calcination-solvothermal method, which greatly improved the photocatalytic performance of the ZnIn₂S₄/g-C₃N₄ heterojunction [20]. However, it is difficult to obtain lattice defects through experiments at the atomic and electronic levels. The calculation method is a supplement to the experimental research and provides detailed preliminary data for the experiment [21]. Therefore, using quantum mechanics or first principles to calculate the performance changes caused by lattice defects is a hot spot [22,23].

In this work, based on the first principles calculation of density functional theory (DFT), we studied the band structure changes of monolayer ZnO and WSe₂ before and after vacancy generation, and systematically studied the vacancy formation energy, band structure, density of states, electronic density difference and optical properties of ZnO/WSe₂ heterostructure before and after vacancy generation.

2. Computational Methods

In this study, we use first-principles density functional theory (DFT) [24] to study the properties of ZnO/WSe₂ heterostructure. The process is implemented by Cambridge Studio Total Energy Package (CASTEP) code [25] from Materials Studio. In order to ensure the accuracy and reliability of the calculation, we first conduct the convergence test of ZnO and WSe₂. Then, according to the comparison of the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) [26] and Heyd–Scuseria–Ernzerhos hybrid functional (HSE06) [27] in Fafei Hu [28,29], for ZnO/WSe₂ heterostructure, the bandgap calculated by HSE06 is larger. Our main research is the effect of different vacancy defects on the ZnO/WSe₂ heterostructure. Therefore, we choose the GGA-PBE exchange-correlation functional to describe the exchange-correlation interaction [30,31,32] under the premise of considering the calculation accuracy and calculation efficiency at the same time. We use the TS method for DFT + D correction to modify and correct the accuracy of the standard GGA-PBE function for the system, because the standard PBE function cannot well describe the vdW interaction. The K point of 6 × 6 × 1 and the plane-wave kinetic energy cutoff of 440 eV are set for the geometry optimization. The Broyden-Fletcher-Goldfarb-Shanno(BFGS) [33] minimize-er was used to optimize the configured geometry to relax the crystal structure and atomic coordinates. The self-consistent convergence accuracy is set to 10⁻⁵ eV per atom, and the convergence criterion for the atomic force is 0.03 eV/Å. Considering that the w element is heavy, spin orbit interaction is used. The vacuum layer was set as 14 Å to minimize the interlayer interaction between periodic layers.

The formula of formation energy $E_f$ for the vacancy in monolayer ZnO, WSe_2, and ZnO/WSe₂ heterostructure can be expressed as [34,35]:

$$E_f=E_{defect}\left(X\right)-E_{perfect}+{\mu }_0\left(X\right)$$

(1)

where $E_{defect}\left(X\right)$ is the total energy of structure containing defective atom X, $E_{perfect}$ is the total energy of perfect structure and ${\mu }_0\left(X\right)$ correspond to the chemical potentials of X atom.

In order to evaluate the stability of ZnO/WSe₂ heterostructure, according to the definition of binding energy (E_b) [36]:

$$E_b=E_{{ZnO/WSe}_2}-E_{ZnO}-E_{{WSe}_2}$$

(2)

where $E_{{ZnO/WSe}_2}$, $E_{ZnO}$ and $E_{{WSe}_2}$ are the total energy of ZnO/WSe₂ heterostructure, a monolayer of ZnO and WSe₂, respectively.

Based on the frequency-dependent dielectric function, the optical absorption coefficient α(ω) can be calculated by the following formula [37]:

$$\alpha \left(\omega \right)=\sqrt{2}\omega {\left[\sqrt{{\epsilon }_1^2\left(\omega \right)+{\epsilon }_2^2\left(\omega \right)}-{\epsilon }_1\left(\omega \right)\right]}^{\frac{1}{2}}$$

(3)

where, ${\epsilon }_1$ and ${\epsilon }_2$ are the real and imaginary parts of the dielectric function, respectively.

3. Results and Discussion

3.1. Structure and Stability

A bulk model of ZnO and WSe₂ (Zinc blende Structure) was first established, then single molecule models of ZnO (001) and WSe₂ (001) were obtained after optimization according to the above parameters, and finally a ZnO/WSe₂ heterojunction model was constructed. The possible vacancy positions of monolayer ZnO, WSe₂ and ZnO/WSe₂ heterostructure are shown in Figure 1. After optimization, the surface thickness of ZnO is 0.635 Å, the surface thickness of WSe₂ is 1.68 Å, and the lattice constants of ZnO and WSe₂ are 6.410 Å and 6.651 Å, respectively. The calculated lattice mismatch is 3.7% which is less than 5%, so it is possible to build heterostructures of WSe₂ and ZnO without inducing structural defects. In order to evaluate the stability of the ZnO/WSe₂ heterostructure, the binding energy (E_b) of the ZnO/WSe₂ heterostructure is −1.276 eV according to Formula (2). It shows that the ZnO/WSe₂ heterostructure can exist stably. According to Formula (1), the formation energy E_f for the vacancy of ZnO−V_O and ZnO−V_Zn, WSe₂−V_W and WSe₂−V_Se, ZnO/WSe₂−V_Zn, ZnO/WSe₂−V_O, ZnO/WSe₂−V_W, and ZnO/WSe₂−V_Se heterostructure are calculated respectively, and the results are shown in

Table 1. The formation energy Ef (eV) for the vacancy in monolayer ZnO, WSe₂ and ZnO/WSe₂ heterostructure. V_Zn means Zn vacancy is generated; similarly, V_O, V_W and V_Se mean O, W and Se vacancy are generated.

	Structure	ZnO	WSe2	ZnO/WSe2
Vacancy
VZn		6.44		−2.56
VO		8.13		7.90
VW			7.66	10.59
VSe			3.40	0.40

. According to the calculation results, except for E_f (ZnO/WSe₂−V_Zn), the rest E_f is positive, which indicates that all the structures can exist stably except ZnO/WSe₂−V_Zn heterostructure. The formation energy E_f for vacancy of ZnO/WSe₂−V_Se is the smallest, which indicates that the structure of ZnO/WSe₂−V_Se is the easiest to form. For ZnO/WSe₂−V_Zn, we also hope to study its structure and properties, find out the reasons for its structural instability, and compare and analyze it with other similar structures.

3.2. Electronic Properties

The energy band structures of monolayer ZnO, ZnO−V_O and ZnO−V_Zn are shown in Figure 2. By comparison, it can be seen from the figure that monolayer ZnO is a semiconductor with a direct band gap of 1.639 eV, and the results are similar to 1.69 eV in the literature [28]. When Zn vacancies are generated, it is interesting that ZnO transforms from semiconductor to metal, showing obvious metallic properties. When the O vacancy is generated, the monolayer ZnO remains a direct bandgap semiconductor, but the bandgap increases to 2.156 eV.

The band structures of monolayer WSe₂, WSe₂−V_W, and WSe₂−V_Se are shown in Figure 3. By comparison, it can be seen that the monolayer WSe₂ is a semiconductor with a direct bandgap of 1.322 eV, and the results are similar to 1.92 in the literature [29]. When W and Se vacancies are generated, the transition from direct to indirect bandgap semiconductors is caused, and the indirect bandgaps are 1.427 eV and 1.363 eV, respectively. The valence band of WSe₂−V_Se is more densely distributed, while WSe₂−V_W produces an empty band of about 1 eV in the valence band.

The band structure and density of states (DOS) of ZnO/WSe₂ heterostructure, ZnO/WSe₂−V_Zn, ZnO/WSe₂−V_O, ZnO/WSe₂−V_W, and ZnO/WSe₂−V_Se are shown in Figure 4 and Figure 5, respectively. It can be seen from Figure 4 that ZnO/WSe₂ heterostructure is a semiconductor with a direct bandgap of 1.363 eV, and the results are also similar to those in the literature [28]. When the Zn vacancy is generated, the direct bandgap of ZnO/WSe₂ heterostructure increases to 1.504 eV; when the O vacancy is generated, the valence band crosses the Fermi level, and the transition from semiconductor to metal takes place, showing obvious metal behavior, which is opposite to that of ZnO−V_O and ZnO−V_Zn in monolayer ZnO. When W and Se vacancies are generated, the transition from direct to indirect bandgap semiconductors occurs, and the indirect bandgap is 1.168 eV and 1.729 eV, respectively. In addition, all the vacancies except ZnO/WSe₂−V_O will cause the unoccupied state above the Fermi level, that is, the gap in the conduction band near the Fermi level. The local magnetic moment appears and is reflected by the spin polarization in the corresponding band structure due to a vacancy in the heterostructure.

It can be seen from Figure 5a–e that the total density of states (DOS) of the heterostructure decreases after the vacancy is generated. That is because the total DOS of the heterostructure decreases after the loss of an atom. The density of states of ZnO/WSe₂−V_O heterostructure is beyond the Fermi level, showing obvious metallic properties. Generally speaking, for the same structure, the lower the density of states at the Fermi level, the more stable the structure is [38]. Comparing with Figure 5b,d,e, it can be seen that the DOS at the Fermi level of ZnO/WSe₂−V_Zn is the highest, which indicates that its structure is unstable; The lowest DOS at the Fermi level of ZnO/WSe₂−V_Se indicates that it is more stable, Which is the same as the conclusion obtained from the calculation of vacancy formation energy in Section 3.1.

At the same time, we compared the electron density difference before and after vacancy generation, as shown in Figure 6. Blue represents charge depletion and red represents charge accumulation. It can be seen from the figure that there is a certain rule of charge depletion and accumulation. There is charge depletion near Zn and W atoms and a large amount of charge accumulation around O and Se atoms. Among them, the charge distribution around each monolayer atom in ZnO/WSe₂ heterostructure is similar, and there are charge depletion and charge accumulation between monolayers. After the relaxation, due to the absence of a Zn atom, the not bound O atom moves to the WSe₂, resulting in a great change in the structure of the ZnO. At the same time, the charge accumulation around the O atom also leads to an increase in the charge depletion around the W atom. That may be the reason why the vacancy formation energy of ZnO/WSe₂−V_Zn is negative. However, the formation of the O, W, and Se vacancies has little effect on the structure of each monolayer, and the charge distribution is regular. Among them, the formation of the O vacancy has the least effect on the structure and charge distribution; After the Se vacancy is generated, the interlayer spacing between WSe₂−V_Se and ZnO is greatly reduced, the charge accumulation and consumption between layers are increased, and the interlayer bonding is more stable, which is the same as the calculation results in Section 3.1. vacancies will change the electronic structure of ZnO/WSe₂ heterostructure and then affect the optical properties. Therefore, it is necessary to study the optical properties of ZnO/WSe₂ heterostructure.

3.3. Optical Properties

It is very necessary to study the optical properties of heterojunction, because optical properties play an important role in photoelectric detection equipment and electronic equipment [39,40]. It is proved that the heterojunction formed by coupling two different semiconductor materials will improve the optical properties of semiconductor materials [41,42]. Therefore, we studied and compared the optical properties of ZnO/WSe₂ heterostructures before and after vacancy generation, including dielectric function and optical absorption coefficient, as shown in Figure 7. According to Formula 3 in Section 2, the frequency dependent dielectric function shows how the incident light interacts when propagating in the material [43].The real part and imaginary part describe the dispersion and absorption effects respectively, i.e., $\epsilon \left(\omega \right)={\epsilon }_1\left(\omega \right)+i{\epsilon }_2\left(\omega \right)$ [44,45]. The imaginary part is related to the energy dissipation entering the medium, in other words, it is related to the photon absorption in the compound.

Figure 7a is the imaginary part of dielectric function. It can be seen from the figure that after the vacancy of ZnO/WSe₂ heterostructure is generated, the imaginary part of the dielectric function has changed greatly, especially in the infrared light region (λ ≥ 700 nm). When the vacancy of O and W is generated, the imaginary part of the dielectric function increases greatly, while the value of it decreases after Zn and Se vacancy is generated. Among them, ZnO/WSe₂−V_W heterostructure changes most obviously. According to the analysis in Figure 7b, the ZnO/WSe₂ heterostructure has absorption peaks in the ultraviolet region, visible region, and infrared region. When λ is about 500 nm, the optical absorption peaks of V_Zn and V_O are slightly larger than that of ZnO/WSe₂ heterostructure, indicating that the visible light absorption of heterostructure is enhanced after the formation of Zn and O vacancies; When λ ≥ 1000 nm, the optical absorption coefficients of V_W and V_Se are larger than those of ZnO/WSe₂ heterostructure, which indicates that the absorption of infrared light is enhanced after the generation of W and Se vacancies. In conclusion, the optical properties of the ZnO/WSe₂ heterostructure will be changed due to the generation of vacancies. The generation of O and W vacancies is accompanied by an obvious red shift, and the generation of Zn and Se vacancies is accompanied by a blue shift. ZnO/WSe₂−V_Zn, ZnO/WSe₂−V_O heterostructures can be applied to the absorption and conversion of visible light and become an important photocatalyst; ZnO/WSe₂−V_O, ZnO/WSe₂−V_W heterostructures have good potential and wide application prospects in infrared light conversion, detection, and other aspects. At the same time, we should try to avoid the production of ZnO/WSe₂−V_Se heterostructure.

4. Conclusions

In this work, based on the first principles calculation of density functional theory (DFT), we studied the band structure changes of monolayer ZnO and WSe₂ before and after vacancy generation, and systematically studied the vacancy formation energy, band structure, density of states, electronic density difference and optical properties of ZnO/WSe₂ heterostructure before and after vacancy generation. The results show that the vacancy formation energy of Zn is negative and the others are positive; The band structure and bandgap of monolayer ZnO, WSe₂ and ZnO/WSe₂ heterostructure are changed due to the generation of vacancies. ZnO−V_Zn and ZnO/WSe₂−V_O heterostructure show metallic properties; The generation of certain vacancies leads to the transition from direct bandgap semiconductor to indirect bandgap semiconductor. Through the analysis of the imaginary part of the dielectric function and optical absorption coefficient, the generation of vacancy will lead to the change of optical properties of the ZnO/WSe₂ heterostructure. The generation of O and W vacancies is accompanied by an obvious red shift, and the generation of Zn and Se vacancies is accompanied by blue shift. ZnO/WSe₂−V_Zn, ZnO/WSe₂−V_O heterostructures can be applied to the absorption and conversion of visible light and become an important photocatalyst; ZnO/WSe₂−V_O, ZnO/WSe₂−V_W heterostructures have good potential and wide application prospects in infrared light conversion, detection, and other aspects. In a word, the generation of vacancy changes the electrical and optical properties of ZnO/WSe₂ heterostructure, which provides a feasible strategy for adjusting the photoelectric properties of two−dimensional photoelectric nano devices and has good potential and broad application prospects.