The effects of vanadium absorbed by WS2 monolayer on the electronic, magnetic and optical properties: A first principle study

Accepted Manuscript The effects of vanadium absorbed by WS2 monolayer on the electronic, magnetic and optical properties: A first principle study Ghazal Bishal, Rostam moradian PII:

S2352-2143(18)30298-3

DOI:

https://doi.org/10.1016/j.cocom.2018.e00352

Article Number: e00352 Reference:

COCOM 352

To appear in:

Computational Condensed Matter

Received Date: 28 September 2018 Revised Date:

24 November 2018

Accepted Date: 26 November 2018

Please cite this article as: G. Bishal, R. moradian, The effects of vanadium absorbed by WS2 monolayer on the electronic, magnetic and optical properties: A first principle study, Computational Condensed Matter (2018), doi: https://doi.org/10.1016/j.cocom.2018.e00352. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The effects of Vanadium absorbed by WS2 monolayer on the electronic, magnetic and optical properties: A First principle study Ghazal Bishal1*, Rostam moradian1,2 Department of Physics, Razi University, Kermanshah, Iran.Tel./ Fax: +98 831 4274556

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Nano science and nano technology research center, Razi University, Kermanshah, Iran.

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Abstract

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In this work, the electronic, magnetic and optical properties of the vanadium doped WS2 monolayer (WS2-V) by using the ab initio calculations are studied. The structure of the pure WS2 suggest a non-magnetic semiconductor with the energy gap of about 2eV. By absorbing the vanadium atom, the material becomes a half-metal with a complete magnetic polarization which has a spin-flip gap of 0.28eV in the down spin state. Vanadium doping induces a big magnetic moment to this structure. The greatest effect on changing the electronic properties of this case is due to d-orbitals of vanadium atoms. Thus, this material can be a good candidate for application in spintronic devices. In the next step, the optical properties of pure and vanadium doped WS2 monolayer are studied in the x and z directions (on the WS2 plane and perpendicular to it respectively). Our results show that at low frequencies, absorption of vanadium on WS2 monolayer in the x-direction has a metallic feature while in the z-direction, it is negligible with respect to the x-direction. Also, reflection in the x-direction is more than z-direction at low frequencies.

1. Introduction

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Keywords: transition metal dichalcogenides, half-metals, electronic properties, optical properties, density functional theory

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Recently, two-dimensional transition metal dichalcogenides (TMDCs) such as MoS2, MoSe2, WS2 and WSe2 have attracted researchers’ attention due to their layered structure, unique electronic and optical properties, and their potential applications in nano-systems[1-6]. Unlike the semi-metal graphene with zero band gap, these materials have direct band gap in mono-layer case, in which the valence band maximum (VBM) and the conduction band minimum (CBM) are placed at the K (K ') point of the hexagonal Brilouin zone[7-12]. Similar to graphene and h-BN, 2D-TMDCs are made not only by mechanical and chemical laminating of layered bulk counterparts but also directly by the chemical vapor deposition (CVD) or two-step thermolysis[13-19]. Just like their bulk counterparts, these 2D materials have different electronic properties covering superconductors[20-21], metals[22-23], insulators[24], and wide band gap semiconductors[25]. In these structures, due to the layered structure and the existence of a wide band gap, it is possible to form layered compounds with different atoms and molecules between the layers of transition metal dichalcogenides[26]. Moreover, various nano-electronic and photonics applications have been proposed for these 2D materials[27-30]. In order to control the band gap of these materials for the construction of optoelectronic devices, researchers have mixed semiconductors with different band gaps and similar atomic structures and thus, they achieved mixed alloy systems with tunable band gap. The acquisition of semiconductor

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nanostructures with adjustable band gap for applications in nano-electronics and nano-photonics is very important. Recent developments in 0D, 1D semiconductor structures have shown that their band gap and light emission can be slowly adjusted by changing their constituent stoichiometries[31].In order to use these materials in the construction of electronic nano scale parts, we can change the energy band gap by absorbing or doping them with especial atoms or molecules. Two-dimensional dichalcogenide semiconductors, including MoS2, MoSe2, MoTe2, WS2,and WSe2 have been the subject of many studies as materials with distinct photo-electrical and optical properties. Also, promising conclusions have been obtained for a wide range of applications in optoelectronic and solar cells using transition metal chalcogenide crystals (TMC) [32].

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On the other hand, one of the most commonly used materials in spintronic devices are the halfmetals[33-35]. Half-metals are materials which act as conductors for one of spin directions while acting as an insulator or semiconductor for the other. Although all half-metals are ferromagnetic, most ferromagnetic materials are not half-metal. Many of the well-known half-metal samples are oxides, sulfides, and Heusler alloys[36,37].

2.Calculation methods

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In this work, by absorbing vanadium atom as an impurity on a single layer unit cell of pure WS2in equilibrium position, we investigate its electronic, magnetic and optical properties, which causes a dramatic transformation in this structure. In the second section, details and calculation methods are given. In the third section, after optimizing the input parameters, the electronic and magnetic properties are studied, and in the fourth section, we examine the optical properties of this material in two different directions, and finally, in the fifth section, we present the summary and conclusion.

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In this work, all the calculations are performed self consistently with the augmented plane waves plus local orbitals (APW+lo)[38,39] method, employing the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA)[40]within the density functional theory (DFT)[41-42] as embedded in theWIEN2k code[43]. The optimized input parameters such as RKmax, Kpoint, and Gmax are chosen to be 8, 3000, 14 respectively.To obtain the optimal structures, the forces on the atomic position are relaxed to 1(dyn/a.u.) in the miniposition command. Moreover, the optical properties are calculated using RPA approximation [44,45]. Transition metals dichalcogenide (TMDC) are layered compounds in which X-M-X atoms are attached together in each layer with strong covalent bonds, and each layer with its adjacent layer has a weak Vander Waals bond. In single-layer case, the transition metal dichalcogenide (TMDC) consists of a hexagonal arrangement of transition metals (M = Ti, Nb, Ta, Mo, W) which is sandwiched between two layers of chalcogen atoms (X = S, Se, Te) [18].We placed the vanadium atom in four possible positions, and the lowest energy of the system referring to the very balanced one belongs to the present position. On the other hand, we could not place it on tungsten positions, since tungsten is tetravalent and it has four covalent bonds with its nearest neighbors, so it can not contribute to new bonds. Moreover, in the case of sulfur, since it is bivalence and vanadium is trivalent, one excess electron remains in the system leading to the

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metallic property, which is in contrast with our goal of making a magnetic half-metal. In Fig.1, we introduce a side and top view of crystalline structure of vanadium doped WS2 monolayer.

3.The electronic properties

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Fig.1Crystal structure of vanadium doped WS2 monolayer, left and right panels are side and top view respectively.

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Our goal is to create magnetization in non-magnetized WS2 monolayer. The vanadium atom is a 3d transition metal with electronic configuration of 4 3 , being one of the best candidates for this purpose. Since it is the only trivalent atom among the transition metals which its atomic radius is very close to that of tungsten atom, and also, 3 spin up valence electrons of vanadium by forming covalent bonds with spin down of 3 sulfur atoms lose their freeness and does not contribute to the conduction process. Hence, for each sulfur atom, one un-bonded spin up electron remains which leads to metallic property of spin up and semiconducting for spin down. Other transition metals such as 26Fe, 25Mn, 24Cr lead to favorable magnetic metals. With this process, we could obtain a half-metal with a high integer magnetic moment which is very useful in spintronics devices. Fig.2 shows the total electronic density of states curve of (a) pure and (b) vanadium doped WS2 monolayer for the spin-up and down states.

Fig.2 (a) and (b) illustrate spin-up and down total DOS of pure and vanadium doped WS2 monolayer respectively.

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Fig.2a shows the pure WS2 graphene-like monolayer that is a semiconductor with an energy gap of 2eV which is in a strong adaptation with other results [10]. Also, spin-up and down density of states curves of the pure WS2 monolayer are quite symmetric indicating the non-magnetic behavior of this structure. The impurity states not only create states inside the spin-up energy gap but also moved conduction edge into the lower energies inside the gap such that, the material becomes a metal in the spin-up state. From Fig.2b, the energy distance between Fermi energy and the lower edge of the conduction band is 0.28eV in the spin-down state which is known as spin-flip energy. In addition, the system is a magnetic half-metal with magnetic moment 5 . To explain the total DOS in detail, we calculated the partial DOS of W, V and S effective orbitals to identify the contribution of each of them in vanadium doped WS2 monolayer (Fig.3). From this figure, we see that most of the states which fill the spin-up energy gap belong to the d-orbital of vanadium (V),although there is a minor contribution of tungsten (W) d-orbitals and sulfur (S) porbitals which are not appeared in the pure WS2 monolayer. These results show that doping V induce a big magnetic moment on V and small magnetic moments on W and S atoms. These magnetic moments are listed in Table.1 Table.1 Magnetic moment ( Magnetic moment

Tungsten(W) 0.10276

Sulfur(S)

0.01406

Vanadium(V) 2.68658

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WS2-V monolayer

Total (super cell) 5.000

) for the vanadium doped WS2 monolayer

Fig.3 The partial DOS of vanadium(V) d-orbitals, tungsten(W) d-orbitals and Sulfur(S) p-orbitals.

In Fig.4, the band structure of (a) pure and (b) vanadium doped WS2 monolayer for the spin-up and down states are presented.

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Fig 4. Total band structure of (a) pure and (b) vanadium doped WS2 monolayer for the spin-up and down states.

4.Optical properties

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As can be seen in the pure case, the system shows a semiconducting property. The spin resolved band structures of vanadium doped WS2 monolayer show a metallic property in spin up states and a semiconducting property in spin down states which is in good agreement with density of states results. Another interesting point in these band structures is that the population of bands in both spin states increases in vanadium doped WS2 monolayer which indicates increasing conducting property in comparison with WS2 monolayer.

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The complex dielectric function ε(ω) is a function of the optical response of the medium to the electromagnetic field, consisting of two real, ε1(ω), and imaginary, ε2(ω) parts. This function is a 3 × 3 tensor in which ε(ω) elements are in the main diameter of this tensor. Our work is based on the RPA approximation [44,45] and in the mentioned tensor on its main diameter, we have xx = yy ≠ zz.

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Fig.5 shows the real part of the dielectric function, ε1(ω), of WS2 monolayer structure with and without vanadium absorption in two x (in the WS2 plane) and z (perpendicular to WS2 plane) directions.

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Fig 5.The real part of the dielectric function of the WS2 monolayer. Both x and z directions are plotted. The red and blue lines denote the states with and without vanadium absorption respectively.

In the pure state, the static value ε1(ω=0) in the x and z directions is equal to 6.2 and 4 respectively, which shows the semiconducting behavior of this structure in both directions, however, we have the semiconducting state with a more broaden gap in the z-direction. The positive and large amount of the static ε1(ω) in x-direction for V doped WS2 monolayer represents its metallic behavior in the infrared region. Besides, if the photon is transmitted perpendicular to the surface of this structure, its static value is 6.2, which suggests the semiconducting behavior in this direction.

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However, with increasing the energy of incident photon along x-direction, the response of matter sharply increased for both structures. In the edge of the visible area, we see an increase in this response. At the energy about 2.5eV, we see a peak for both structures, so that the peak of the pure case is slightly higher than the doped one. By increasing the energy of the photon, the response along x-direction decreases, while some peaks are observed around 3.3eV and 4.1eV.

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In the energy range of 6.8eV to 8.4eV, as well as higher energies in the region of 11.4eV to 15.9eV, the ε1(ω) becomes negative along the x-direction. That is, at these intervals, the photon does not pass through the material and we observe the highest amount of reflection in these regions. However, above 16eV, ε1(ω) has less response to the incident photon than in the visible area which is almost zero. In these regions, the V doped WS2 monolayer exhibits an insulator feature, so ε1(ω) above 7eV for both pure and V doped WS2 monolayer, the material response to an incident photon is negligible with respect to other energy ranges. Also in z-direction, with increasing energy of the incident photon in the pure state up to the limit of 2eV (the energy gap range), ε1(ω) is almost constant, which is why no electron is able to travel the bandwidth of this compound, and then, with the increase in the energy of the incident photon, the response to the incident photon at two energies of 4eV and 5.4eV is coupled with two peaks, and the second peak is higher than the first one. In the V doped WS2 monolayer, the material response in the energy of 1eV is coupled with a peak and then reduced, but after the energy of 4eV, the behavior is the same as the pure case, with a difference that the peak intensity is lower

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than that of the pure case. However, by increasing the photon energy and in the range of energies higher than 5eV for both structures, the amplitude of the response to the incident photon in both directions decreases and the major decrease is observed in the energy range of 10eV. In zdirection, for the two energy ranges of 6.4 to 7eV and 9 to 17eV, we see that ε1(ω) becomes negative, i.e. in these energy regions, these materials behave as absolute reflectors and no transmission is observed. At the energies of 8.4, 15.9eV in x-direction, and 7, 17eV in zdirection,ε1(ω) becomes zero representing the plasmonic frequencies.

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The peaks of ε2(ω) curve represent the electron transition from the occupied levels to the empty levels. Fig.6 shows the ε2(ω) curve in x and z directions for the pure WS2 and V doped WS2 monolayers. It is clear that the behavior of optical transitions is different in the infrared, visible and ultraviolet areas. For both x and z direction of the pure WS2, from the energy 0eV to about 2.5eV (this amount is smaller for x-direction), the ε2(ω) doesn't have any states and we see an optical gap which is equivalent to the WS2 electronic gap.

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Fig 6.The imaginary part of the dielectric function of the WS2 monolayer. Both x and z directions are plotted. The red and blue lines denote the states with and without vanadium absorption.

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After 2.5eV, we see a very strong increase in the ε2(ω) curve for the pure case; this increment for x-direction is greater than z. The peaks after 2.9eV and 4.5eV for x and z directions respectively are the indication of the first transition from the valence to the conduction band of this compound. But in the case of V doped WS2 monolayer at x-direction, the ε2(ω) curve has a large amount at very low energies due to the intraband transitions showing the metallic nature of the compound. Similarly, in the z-direction at low energies, we observe that the ε2(ω) curve has a peak, but its value is low, which indicates a semi-conductivity state close to the metal in this direction. Along x-direction at the edge of the visible area, the ε2(ω) curve shifts towards a minimum, and by increasing the energy of the incident photon, we find that there are several main peaks in energies of 2.8eV and 4.5eV. In V doped WS2 monolayer along the z-direction, the first main peak appears at the energy 1.2eV, and then slightly reduces until 2.5eV.After this region, we see a sharp increase in this curve with a steep slope, and the most electron transitions

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from valence to the conduction band occur in the energy 6eV. Then, there is a sharp decrease in the curve slope and another major peak in the energy of 9.5eV is observed, and then, the curve decreases with a slight gradient and tends to zero.

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The optical energy loss spectrum (Eloss) represents the amount of energy loss of the incident photon in the material. Fig.7 shows the Eloss diagram for both compounds in the two mentioned directions.

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Fig 7. The calculated energy loss function, L(ω), of pure and V doped WS2 monolayers for x and z directions. The red and blue lines denote the states with and without vanadium absorption.

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For the pure case, in both x and z directions, there exists no state at energy gap range. This means that the pure WS2 monolayer is a semiconductor in both directions. But for V doped WS2 monolayer along x-direction, dissipation exists even at zero energy implying the metallic properties of this compound in this direction. In z-direction, after a small energy range, the loss peak is detected which means that the material in this direction is a semiconductor with a small energy gap. According to Fig.7, the maximum energy loss belongs to the energy region of 15eV to 19.5eV which corresponds to the plasmonic frequencies. This situation is observed for both x and z directions in both structures, but the loss intensity is sharper for z-direction with respect to the x-direction, while in x-direction the peak is broadened. Fig.8 shows the optical absorption curve of WS2 and V doped WS2 monolayers in x and z directions. For pure case up to 2.5eV, there is practically no absorption in both directions, but with increasing the energy of the incident photon in both x and z directions, the absorption curve is strongly increased. In the case of V doped WS2 monolayer, a negligible absorption is observed in the infrared region for x-direction, while a peak is observed after a small gap of about 0.3eV in the z-direction, this means that these materials have no photon absorption in low energy ranges, because it shows a metallic behavior in x-direction and acts as an absolute reflector. Also, it is a semiconductor in z-direction and has no electron transitions in energy ranges lower than its spin-

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flip band gap. At energies above 2.5eV for both directions of radiation, the two structures are almost identical, and their peaks correspond to the peaks of ε2(ω).

Fig8.Spectral dependence of the absorption coefficient, α(ω), of the WS2 monolayer. Both x and z directions are .plotted. Red and blue lines denote the states with and without vanadium absorption

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Fig.9 shows the real part of refraction curve in x and z directions for pure and V doped WS2 monolayers.

Fig9. The calculated refractive index n( ) of the WS2 monolayer. Both x and z directions are plotted. The red and blue lines denote the states with and without vanadium absorption.

The static value of the refractive index in the pure case is 2.5 and 2 for x and z directions respectively, and these are in good agreement with the root of static value of the real part of

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dielectric function (N= 0 ). Also, for each direction particularly z, this value is uniform until the edge of the electronic energy gap. In x-direction, in the range of 1.5eV to 4.5eV, where the electron transition occurs (Fig.6) and ε1(ω) has a peak (Fig.5), a relative increase in the refractive index is observed. But then with increasing the energy of the incident photon, the amplitude of the refractive index decreases with a steep gradient, so that after the energy of 11.0eV, the refractive index takes a value less than one, and then the structure plays the role of an accelerator for the incident photon representing the superluminal effect. The same phenomena occur in zdirection where an increase of the refractive index is started from 2.5eV so that the peaks are in the region of 2.5eV to 10eV, and the superluminal phenomenon occurs after 10eV.

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The only difference between the pure and V doped WS2 monolayers is at low energy ranges and in the infrared region. Along x-direction and in very low energies near the static state, there is a major peak, which again confirms the crystal conductivity in this direction. Along z-direction, this coefficient equals to 2.5 indicating the semi-conductivity feature.

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In the end, the curve of reflectivity is depicted in Fig.10 for the pure and V doped WS2 monolayers in both x and z directions. In metals, the static value of the reflection index is high (almost more than 90%), which is observed for V doped WS2 monolayer in x-direction.

Fig 10. The calculated frequency-dependent reflectivity of the WS2 monolayer. Both x and z directions are plotted. The red and blue lines denote the states with and without vanadium absorption.

The static value of the reflection index for the pure case in both x and z directions is less than 0.2, which means that the material behaves like a semiconductor in these directions. Whereas, this value for V doped WS2 monolayer is less than 0.2 in the z-direction, its semi-conductivity is confirmed which is more than 0.5 in the x-direction indicating its metallic behavior. In higher energies, the maximum reflection coefficient for both diagrams is less than 0.4 which is a small

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number, this means that at the best situation, it reflects 40% of the incident photon and absorbs or allows to pass the rest of it. Comparing Fig.6 and Fig.8, it is observed that most of the incident photons pass through the material or are spent on the electron transitions. Therefore, in the range of energies above 20eV,unlike the low energies, this material is transparent, and this means that by changing the energy of the incident photon, we see different optical behaviors.

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Also, in both compounds, we see the convergence of the ε2(ω) curves after 17eV which tend to zero in both directions. Considering this region of energy, as shown in Fig.10, the amount of reflection of the photon is diminished. It is also concluded that in energies above 22eV, the photon completely passes through the material in both cases so the material becomes transparent. Similarly, at high energies, the real refractive index (Fig.9) shifts to values of about 1, which means that the object behaves like a vacuum environment and practically no refraction occurs for the incident photon. 5. Summary and conclusion

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Using the density functional theory and WIEN2K code, we investigated the electronic, magnetic, and optical properties of the WS2 monolayer in two pure and vanadium doped cases. In the pure case, an energy gap of 2eV is obtained for this material suggesting a non-magnetic semiconductor. By absorbing V atom, the V doped WS2 monolayer is a conductor for spin-up and a semiconductor with 0.28eV spin-flip energy gap for spin-down states. Doping induces a big magnetic moment on V and small magnetic moments on W and S atoms in V doped WS2 monolayer. It becomes a full half-metal with a complete magnetic polarization, the most effect on changing the properties of this material is due to the d orbital of W and V atoms making this material a potentially good candidate for the spintronic devices. The optical properties of pure WS2 and V doped WS2 monolayers in x and z directions (in the WS2 monolayer plane and perpendicular to it respectively) are investigated by discussing the diagrams of real and imaginary parts of the dielectric function (ω) and (ω), the Eloss function, the absorption curve, the real part of refractive index n(ω) and reflectivity. According to the (0) graph (in the static state), the pure WS2 monolayer is a semiconductor in x and z directions, with a predicted wider gap in the z direction. The V doped WS2 monolayer at the zero frequency ( = 0) along xdirection has completely metallic features, but shows semi-conductivity in the z-direction. Most response to the incident photon is observed in the visible area for pure and V doped WS2 monolayer in both directions. In the energy ranges of 6.8eV to 8.4eV and also 11.4eV to 15.9eV, the real part of dielectric function 1 (ω) along x-direction is negative for two structures, and photon does not pass through the material being in agreement with the peaks of the reflectivity index. We have the same situation for the z-direction in both structures at the energy ranges of 6.4eV to 7eV and 9eV to 17eV. In two x and z directions for the pure WS2 monolayer, from 0eV to about 2.5eV (it is a some smaller number for x direction), we see an energy gap in the curve of ε2(ω), which is proportional to the electronic band gap of the WS2 monolayer. The most energy loss occurs at the energy region of 15eV to 19.5eV in both structures. The curve of reflection index for both cases in both directions shows that in the range of energies above 20eV, in contrast to low energies, this materials are transparent, which means that different optical behaviors are observed by changing the direction of incident photon energy.

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