1 Introduction
In recent years, the zinc sulfide (ZnS) crystal has been a research focus due to its attractive physical properties [1-4] as well as its promising device applications [5-12]. It has been widely used in diverse fields including semiconductors, thin film electroluminescent, sensors, and so on. Silver activated ZnS (ZnS(Ag)) has a maximum in the scintillation emission spectrum at 450 nm. The light conversion efficiency is relatively poor for fast electrons which may be an advantage when detecting heavy charged particles in a relatively intense γ-ray background [13]. ZnS(Ag) can also be used to detect thermal neutrons if a lithium compound enriched in 6Li is incorporated [14]. Another use for ZnS(Ag) is for the detection of fast neutrons [15]. A fast neutron detector can be made by imbedding the ZnS(Ag) in a clear hydrogenous compound. ZnS is usually used as a scintillation cell component for the detection of the alpha particles [16-20].
There are two types of structures of ZnS, the wurtzite (WZ) structure and the zinc-blende (ZB) structure. There are a few researchers that made contributions to the study of WZ ZnS. Chen [21] et al. investigated the optical and excitonic properties of the ZnS nanowires toward efficient ultraviolet emission at room temperature. Reddy [22] et al. found that WZ ZnS is more desirable for optoelectronics because its luminescent properties are considerably more enhanced than sphalerite. Zeng [23] et al. investigated charge transfer and optical properties of wurtzite-type ZnS/(CdS/ZnS)n (n = 2, 4, 8) superlattices. Zhang [24] et al. investigated the blue and green emissions of ZnS ceramics and found that the wurtzite-type ZnS ceramics could provide an interesting application in the development of novel luminescent devices. Ong [25] et al. determined optical constants of WZ ZnS thin films by spectroscopic ellipsometry. Zhang [26] et al. researched the surface states and their influence on the luminescence of ZnS nanocrystallite. Ye [27] et al. investigated the origin of the green photoluminescence for the ZnS. Tadashi Mitsui [28] et al. studied the cathodoluminescence image of defects and luminescence centers in ZnS/GaAs(100). Wang [29] et al. studied the photovoltaic effect of ZnS as semiconductors.
However, there are few theoretical researches on the ZnS as a scintillation cell component, and many researchers [16-20] studied the ZnS crystal as a kind of scintillation material used for alpha particles detection through the experimental study, but they did not mention the crystal structure of the ZnS crystal. As a scintillation component, the electronic structure and optical properties are important to detective performance. In order to explain the mechanism of the scintillation luminescence in WZ ZnS, we need to know the effect of the Ag-doping, Zn vacancy on the detective performance of ZnS, and whether the WZ crystal structure is better as a kind of scintillation material. We studied the lattice constants, electronic structure, and optical properties of the pure ZnS, ZnS(Ag), and VZn.
2 Calculation
The space group of WZ ZnS is P63mc(186), and the crystal structure of the perfect WZ ZnS is shown in Fig. 1(a) and (b). There is one zinc atom and one sulphur atom in each ZnS unit cell, in which the atomic coordinate of Zn is (0.3333, 0.6667, 0), and the atomic coordinate of S is (0.3333, 0.6667, 0.374). We applied a 2×2×1 supercell to calculating the electronic structure and optical properties of ZnS, ZnS(Ag), and VZn. Figure 1(c) is the crystal structure of the ZnS doped with an Ag atom, noted as ZnS(Ag). Figure 1(d) shows the crystal structure of the ZnS with a Zn vacancy, noted as VZn in this paper. The Ag doping concentration and the vacancy concentration may be a bit high, but it is reasonable to explain the effects of the Ag doping and the vacancy on the electronic structure and the optical properties of the WZ ZnS.
All calculations were using the Cambridge Serial Total Energy Package (CASTEP) [30]. CASTEP is an ab initio quantum mechanics application software based on the DFT. It uses the plane wave pseudo-potential (PWP) method to substitute ionic potentials. The expansion of the electronic wave function is spread through the plane wave basis sets. The exchange correlation potentials among the electrons are described by the general gradient approximation (GGA) with a Perdew-Burke-Ernzerhol (PBE) form [31]. To ensure the calculation efficiency and accuracy, after the convergence test, we chose the kinetic cutoff energy as 400 eV for all the calculations, and the k-mesh as 4×4×4. We obtained the equilibrium crystal structure when the maximum force on all atoms was less than 0.05 eV/nm, the maximum stress of the crystal was smaller than 0.02 GPa, and the maximum change of energy per atom was 1.0× 10-5 eV/atom. The valence electrons involved are 3d104s2 for Zn, 3s23p4 for S, and 4d105s1 for Ag, respectively. After we optimized the crystal structure of ZnS, ZnS(Ag), and VZn, we calculated the electronic structure and optical properties of them.
3 Results and discussion
3.1 Optimization of the crystal structure
In order to figure out the electronic structure and optical properties of WZ ZnS, ZnS(Ag), and VZn, we optimized those crystal structure first. Table 1 shows the results of our calculation, the other theoretical results, and experimental data.
From Table 1, lattice parameters of those crystals that we calculated are consistent with the other theoretical results and experimental data. The band gap of WZ ZnS we calculated is 2.17 eV which is consistent with other theoretical values 2.19 eV [32], 2.06 eV [33], while they are smaller than the experimental data 3.7-3.8 eV [1] and 3.91 eV [34] for the WZ ZnS. The underestimation of the band gaps is mainly due to the simple form of GGA, and this is a well-known issue [35]. It should be noted that the density functional formalism is limited in its validity [36] so the band structure derived from it cannot be used directly for comparison with experiment. However, GGA is still a widely used method and has achieved many successes. Results based on GGA are generally acclaimed [37].
3.2 Electronic structure
As a scintillation material, the electronic structure of WZ ZnS is important to its luminescence performance. When ZnS is exposed to the alpha particles radiation, photons are produced by the de-excitation of the electrons. The luminescence performance properties of scintillation materials are closely related to the electronic structure of them. The detection performance will be better with higher luminous efficiency of the visible light photons. We calculated the band structure and the electronic density of states of ZnS, ZnS(Ag) and VZn.
Figure 2 is the band structure of those crystals, which shows the band gaps of WZ ZnS, ZnS(Ag), and VZn are 2.17 eV, 1.79 eV, and 2.37 eV, respectively. Compared with the pure ZnS, ZnS(Ag) changes somewhat because of the doping. The conduction band of ZnS(Ag) is similar to that of ZnS, but the top point of the valence band of ZnS(Ag) moves to a higher energy level. Both the top point of the valence band and the lowest point of the conduction band of VZn move to a higher energy level when compared with ZnS. The top point of the valence band of VZn is still lower than that of ZnS(Ag).
In a word, the main changes occur near the Fermi level when there are Ag doping and Zn vacancy in WZ ZnS. The electrons of ZnS(Ag) need less energy to transit from the valence band to conduction band. Therefore, Ag doping can improve the detection efficiency of ZnS because it can produce more photons when exposed to the same amount nuclear radiation and their energy is in the visible light range. On the contrary, the electrons of VZn need more energy, which means that Zn vacancy decreases the detection efficiency of ZnS.
Figure 3 is the distribution of the total density of states (TDOS) of WZ ZnS, ZnS(Ag), and VZn. From Fig. 3, the result of the TDOSs is in agreement with the band structure. In the low valence band (-13 eV–11 eV), no changes occur. The lowest point of the conduction band changes somewhat in ZnS(Ag) and VZn. Those results can explain the difference of the band gaps.
Figure 4 shows the TDOS and the partial density of states (PDOS) of WZ ZnS. The low valence band of ZnS is composed by the s state electrons of S, and the high valence band is mainly composed by the d state electrons of Zn and p states electrons of S. The forming of the conduction band is the contribution of the s state electrons of Zn and the p state electrons of S.
Figure 5 shows the TDOS and PDOSs of ZnS(Ag). The top point of the valence band of ZnS(Ag) moves to a higher energy level, and the lowest point of the conduction band doesn’t change. The s state electrons of Ag make some contribution to the high valence band, the d state electrons of Ag make some contribution to the conduction band, and the result is the band gap becomes smaller than that of pure ZnS. Compared with the pure ZnS crystal, Ag doping could produce more visible light photons when ZnS(Ag) is exposed to the the same amount of radiation.
Figure 6 shows the TDOS and PDOSs of VZn. Both the top point of the valence band and the lowest point of the conduction band of VZn move to a higher energy level when compared with WZ ZnS. The band gap of VZn is bigger that of ZnS, which means electrons need more energy to transit from the valence band to the conduction band. This result decreases the detection efficiency. Therefore, reducing Zn vacancies is beneficial to improving the detective performance of ZnS.
3.3 Optical properties
In the linear response range, the optical response function of a crystal can usually be described by a dielectric function in a complex number form ε(ω) = ε1(ω) + iε2(ω), from which we can easily get the spectrum information. The imaginary part, ε2(ω), represents absorption characteristic well, and it is associated electronic structure. We calculated ε2(ω) according to the definition of direct probability transition.
Figure 7 shows ε2(ω) of WZ ZnS, ZnS(Ag), and VZn, respectively. In the high energy range (5 eV - 10 eV), the change isn’t obvious. In the low energy, about 1.5 eV, there are two peaks belonging to ZnS(Ag) and VZn, respectively. Those changes indicates that Ag doping and Zn vacancy can enhance the absorption of the ZnS crystal to the low energy photons, and it has a negative effect on the detection efficiency of ZnS(Ag) and VZn.
Figure 8 shows the absorption spectra of ZnS, ZnS(Ag), and VZn. Obviously, Ag doping and Zn vacancy could increase the absorption of low energy photons whose energy is in the visible light range. VZn absorbs more photons than ZnS(Ag) at the same energy.
Figure 9 shows the reflection coefficient of WZ ZnS, ZnS(Ag), and VZn. In the low energy range, around 1.8 eV, we can observe two peaks belonging to ZnS(Ag) and VZn, respectively. They are beneficial to detective performance. Valleys appears around the 2.9 eV where the reflection coefficients of ZnS(Ag) and VZn are smaller than the perfect ZnS. The valleys would weaken the detection efficiency. In short, a red-shift can be observed in the Fig. 9. Ag doping and Zn vacancy increase the reflection coefficient of the low energy photons. A higher reflection coefficient of the low energy photons can improve the detection efficiency of ZnS.
4 Summary
In order to investigate the effect of Ag doping and Zn vacancy on the detective performance of WZ ZnS as a scintillation cell component, we calculated the electronic structure and optical properties of ZnS, ZnS(Ag), and VZn by using the first-principles method based on the DFT. The results of our calculation are as follows:
(1) The band gaps of WZ ZnS, ZnS(Ag), and VZn are 2.17 eV, 1.79 eV, and 2.37 eV, respectively. It shows that electrons of ZnS(Ag) need less energy, and electrons of VZn need more energy than ZnS, to transit from the valence band to conduction band when they are exposed to the same amount of radiation.
(2) The density of states show that the top point of the valence band of ZnS(Ag) moves to a higher energy level, the lowest point of the conduction band doesn’t change, and the result is the band gap becomes smaller than pure WZ ZnS. Both the top point of the valence band and the lowest point of the conduction band of VZn moves to a higher energy level, when compared with pure ZnS.
(3) Ag-doping and Zn vacancy enhance the absorption of the low energy photons, especially in VZn. Reflection coefficient of ZnS(Ag) and VZn perform higher than ZnS in the visible light range. Meanwhile, a specific energy, about 2.9 eV, leading to decrease of detection efficiency is observed, which needs to be avoided.
The results indicate that Ag-doping has a complex effect on the detection performance. It is beneficial to produce more visible light photons than pure WZ ZnS when exposed to the same amount of radiation, because the increase of the absorption to visible light photons weakens the detection performance. Zn vacancy has a negative effect on the detection performance. If we want to improve the detection performance of WZ ZnS, Ag-doping will be a good way, but we should reduce the absorption to visible light photons and control the number of vacancy carefully.
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