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Optoelectronic properties of MoS2-ReS2 and ReS2-MoS2 heterostructures
Journal Pre-proof Optoelectronic Properties of MoS2-ReS2 and ReS2-MoS2 Heterostructures
Muhammad Saeed, Waqar Uddin, Awais Siddique Saleemi, Muhammad Hafeez, Madiha Kamil, Irshad Ahmad Mir, Sunila, Rooh Ullah, Shafiq Ur Rehman, Zhu Ling PII:
S0921-4526(19)30705-7
DOI:
https://doi.org/10.1016/j.physb.2019.411809
Reference:
PHYSB 411809
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
05 September 2019
Accepted Date:
21 October 2019
Please cite this article as: Muhammad Saeed, Waqar Uddin, Awais Siddique Saleemi, Muhammad Hafeez, Madiha Kamil, Irshad Ahmad Mir, Sunila, Rooh Ullah, Shafiq Ur Rehman, Zhu Ling, Optoelectronic Properties of MoS2-ReS2 and ReS2-MoS2 Heterostructures, Physica B: Physics of
Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411809
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Optoelectronic Properties of MoS2-ReS2 and ReS2-MoS2 Heterostructures Muhammad Saeed,a,b Waqar Uddin,c Awais Siddique Saleemi,d Muhammad Hafeez,e Madiha Kamil,f Irshad Ahmad Mir,e Sunila,g Rooh Ullah,h Shafiq Ur Rehman*e and Zhu Ling*e aState
Key Laboratory of Nuclear Resources and Enviromement, East China University of Technology, Nanchang, 330013, People, s Republic of China.
bCollege
of Nuclear Science and Engineering, East China University of Technology, Nanchang, 330013, China
cDepartment dInstitute eCollege
of Chemistry, Khushal Khan Khattak University, Kharak, Pakistan
for advanced study, Shenzhen University, Shenzhen, Guangdong 518060, P. R. Chin
of Physics and Optoelectronic Engineering, Shenzhen University, Nanhai Ave. 3688, Shenzhen, Guangdong 518060, People’s Republic of China
fDepartment gDepartment
of Computer Science, Abdul Wali Khan, University, Mardan, 23200, Pakistan
of Physics, Balochistan University of Information Technology, Engineering and Management Sciences, Quetta, Pakistan
hDepartment
of Chemistry, University of Turbat-92600, Balochistan, Pakistan *Corresponding author email: [email protected] *Corresponding author email: [email protected] Abstract
We have investigated the electronic properties of heterostructures MoS2-ReS2 and ReS2-MoS2 with hybrid density functional theory. Contrary to the reported work, we found that ReS2 is an indirect band gap semiconductor material in the 2H phase, in good agreement with experimental work. Furthermore, the calculated charge density profile and weighted bands show that MoS2ReS2 heterostructures have type-II band alignment whereas ReS2-MoS2 have type-I band alignment with indirect band gaps, which are in good agreement with the literature. In MoS2ReS2 heterostructures, both electrons and holes were positioned in ReS2 layer in the form of an exciton, while in ReS2-MoS2 the electrons and holes were located in different layers which
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separated the electrons and holes into two different regions. The former heterostructure material is useful to the photovoltaic devices and the later is for the application of optoelectronic devices. Key words: Band structure engineering, Type-I and Type-II band alignments, ReS2-MoS2 and MoS2-ReS2 monolayers heterostructures, Density functional theory 1. Introduction In the recent decays, the discovery of new class of two-dimensional (2D) semiconductor monolayer materials such as, MoS2, WS2, MoSe2, Mo2C, ReS2, ReSe2, W2C, TiS2, SnS2 and ZrS2, has become a hot topic for the researchers, due to its potential applications in the field of optoelectronic, energy harvesting, gas sensors, catalysis, field effect transistors and storage devices.1-9 These nanoscale materials have astonishing behavior such as tunable electronic energy gap, high optical absorption, and high mechanical strength, which make them superior in the microelectronic industry.10-12 But for the application to the photonic and optoelectronic devices, a proper direct band gap material is essential. Therefore, scientists are searching for new pathways to tune the properties of these semiconductor monolayers such as by doping, strain engineering, staking, and heterostructure to achieve the required band gap material.13-16 By introducing heterostructure between the two materials, the charge carriers (electrons and holes) can be controlled and a desirable optical energy gap material can be achieved.16-18 According to recent experimental reports, only six kinds of MX2/MX2 (MoS2/WS2, MoS2/WSe2, MoS2/MoSe2, WS2/WSe2, MoS2/MoTe2, and MoSe2/WSe2) monolayer heterostructures were fabricated using mechanical transfer techniques or a direct chemical vapor deposition (CVD) method, many future novel devices like field effect transistor (FETs) are based on these heterostructure between the two materials 19-20. On the bases of band alignment, these heterostructures are classified into two types, symmetric (type-I) and staggered (type-II). In type-I band alignment, both the conduction band minimum (CBM) and valence band maximum (VBM) of a narrow energy gap material are located in the wide energy gap material illustrated in Fig. 1(a) & 1(b). Where electrons and holes in the form of excitons are excited in one material and transfer to the other material across the heterostructure interface. These electrons and holes are afterward recombining in one component of the heterostructure which enriches the optical properties of the material. The symmetric Type-I band alignment is most extensively used in certain optical devices e.g., light emitting diodes (LEDs) and in lasers as they provide a way to spatially, 2
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confine electrons and holes so that efficient recombination can occur.20 The type of band alignment where CBM (electrons) and VBM (holes) are not confined to a specific material but are located in different materials is called type-II band alignment. Type-II band alignment is very useful for photovoltaic and photocatalyst devices, since they allow large band offsets on one side.21-25 Rhenium disulfide (ReS2) monolayer is a new member of 2D semiconductors, which played a vital role in industries for the application of photonic devices.26, 27 Since Rhenium disulfide is a new member of transition metal dichalcogenides (TMDs) family, therefore, it exhibits many exotic properties i.e., direct band gap, strong in-plane anisotropy and weak interlayer coupling.5, 27
ReS2 based field effect transistors (FETs) and digital devices showed tremendous electronic
performance. 28, 29 The electronic properties of ReS2 are still a big challenge, it has been reported that it has a direct band gap (1.5eV) in their bulk form and in the monolayer as well5. On the other side, S. Horzum claims with the help of first principle calculations that ReS2 is metal in both 2H and 1T phase.28 While some other theoretical work has investigated that it is indirect semiconductor in bulk and in monolayer as well.30 Since it is important to clarify the misconception among the researchers about their electronic structure before using it in the devices. In the present work, we clarify this inconsistency by using hybrid density functional theory (HSE06). On the other hand, MoS2 is one of the most studied layer semiconductor material and have a variety of application including solar cell, blue light emitting diodes, electroluminescent displays, and photoluminescence devices, etc.1, 10, 15 Moreover, a negligible lattice mismatch was found between the lattice parameter of MoS2 with ReS2 and (mismatch~0.2%) which makes it more attractive for optoelectronic devices in combination with ReS2. Therefore, experimentalists have discovered a very recent ReS2/MoS2 monolayer heterostructure, an indirect band gap semiconductor with type-II band alignment.19. Similarly, an update experimental report on the van der Waals heterostructure (MoS2/ReS2) formed by monolayer’s of MoS2 and ReS2 has been found in the literature, stated that a type-I band alignment exits in such heterostructure with the conduction band minimum and valence band maximum are situated in the ReS2 layer.20 This configuration is unlike from the previous reported type-II van der Waals heterostructures where holes and electrons exist in different layers. The type-I nature of MoS2-ReS2 monolayer heterostructures is confirmed with an experiment by photo-carrier dynamics and transient absorption measurements.20 3
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Motivated by the above experimental work in these heterostrutures, we employ hybrid density functional theory to predict new monolayers ReS2/MoS2 and MoS2/ReS2 heterostructures and their electronic band structure in 2H phase. Here, we demonstrate type-II band alignment and their transition from type-II to type-I band alignment with slightly different intrinsic electric field environment in these heterostructures. Our main focus will be on recent experimental reports, where they have successfully achieved type-II and type-I band alignment in these heterostructures.19,20 We confirm these findings by theoretical investigations and the most important is the type-I alignment, which will be a new prediction by band structure engineering, which enables the heterostructure materials for the highly efficient optoelectronic devices applications. Further, we will also study structure parameters, band structure, localizations of electrons and holes in both ReS2/MoS2 and MoS2/ReS2 heterostructures by using a well-known density functional theory (DFT) approach with GGA-PBE approximation. However, the problem with the semi-local functionals is that it always gives underestimate band gaps values and unclear electronic properties; therefore for further confirmation of our results we employ hybrid density functional HSE06 calculations.21
Fig.1. Description of the Band alignment between two semiconductors a) Type-I band alignment (left side) and b) Type-II alignment (Right side) 2. Methodology Density functional theory, Generalized Gradient Approximation (GGA) with Perdew-BurkeEnzerhof (PBE), calculations was performed in the first step and then their input was used for the HSE06 calculation. Plane Wave pseudopotential code Vienna Ab-initio Simulation Package 4
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(VASP) was used for these simulations. The Van der Waals interactions (Grimme’s approach) were considered to optimize the geometries as well as to perform further calculations. We first built the bulk structures of MoS2, ReS2 separately in its 2H phase than convert it into pristine monolayers. To construct the MoS2-ReS2 monolayer heterostructures, we insert the monolayer of ReS2 into the MoS2 monolayer lattice and for the ReS2-MoS2heterostructure we insert MoS2 in the lattice of ReS2 monolayer. To prevent the interaction between the monolayers a vacuum of 20Å was apply to the c-direction. The structures were relaxed till the forces and energy reached to 0.0001 eV/Å with a high plan wave cutoff of 500 eV. A K-mesh of 12×12×1 was used for the relaxation. 3. Results and discussion The bulk MoS2 was first to design in its 2H phase, as usually 2D transition metal dichalcogenides exist in two phases i.e. 2H-phase and 1T-phase. MoS2 and ReS2 are found to be energetically stable in its 2H phase, therefore, we consider their 2H-phase in the present work. Afterward, we apply a vacuum of 20Å in the c direction to convert this bulk MoS2 into monolayer and the structures were relaxed. The optimized lattice parameters for MoS2 monolayer (a=b= 3.18Å) and the bond length Mo-S is (2.41Å) and for ReS2 monolayer lattice parameters (a=b= 3.16Å), bond length( Re-S=2.40Å) which were in better agreement with literature.31, 32 Further, the ReS2 layer was induced in the MoS2 lattice in such a way that the S atoms of one layer comes on the top of Mo and Re atoms in the other layer and hence the MoS2ReS2 heterostructure were formed with (AA) stacking as shown in Fig.2(a). The lattice parameters for the MoS2-ReS2 are (a=b= 3.16Å), bond lengths (Mo-S= 2.41Å, Re-S= 2.41Å). Where the lattice mismatch (0.2%) between MoS2 and ReS2 is quite small, therefore, there is no need to apply the supercell mechanism to reduce the lattice mismatch. Similarly, the pristine ReS2 monolayer lattice was first created and then insert the MoS2 monolayer in its lattice by introducing ReS2-MoS2 heterostructure shown in Fig. 2(b). For ReS2-MoS2 lattice parameter (a=b= 3.15Å), bond lengths (Mo-S= 2.40Å, Re-S=2.41Å). Further, to understand the relative stability of the ReS2-MoS2 and MoS2-ReS2 heterostructures the binding energy, Eb can be calculated by the difference in the total energy of the heterostructure and their components. The binding energy formula of ReS2-MoS2 and MoS2-ReS2 heterostructures can be defined as
Eb EMoS2 Re S2 EMoS2 ERe S2 5
(1)
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Eb ERe S2 MoS2 ERe S2 EMoS2
(2)
Equation (1) represents MoS2-ReS2 while Equation (2) represents ReS2-MoS2 heterostructures. Where EMoS2 Re S2 and ERe S2 MoS2 represent the total energies of ReS2-MoS2 and MoS2-ReS2 heterostructures per unit cell EMoS2 and ERe S2 are the total energies per unit cell of the individual layers. The calculated binding energies are -0.31, -0.32, -0.34, and -0.37eV for ReS2, MoS2, MoS2-ReS2 and ReS2-MoS2, respectively. Where the negative binding energies of the heterostructures confirm that the two heterostructures are energetically stable. The binding energy of MoS2-ReS2 is lower than ReS2-MoS2 monolayer heterostructures which indicate that the MoS2-ReS2 heterostructure is comparatively more stable than ReS2-MoS2. To check the stability of ReS2-MoS2 and MoS2-ReS2 monolayer heterostructures in the terms of electronic charge distribution, the 3D charge density was calculated by subtracting the electronic charge of ReS2-MoS2 and MoS2-ReS2 heterostructures from their corresponding pristine monolayers i.e. MoS2 and ReS2. Fig.1a shows that there is no charge transfer at the interface of ReS2-MoS2 and hence the heterostructures is less stable because of no strong binding between the two monolayers. On the other hand, the charge density is spread at the interface for MoS2-ReS2 which confirm strong binding between MoS2 and ReS2 and hence MoS2-ReS2 is stable than ReS2-MoS2 hetrostructure. 3.1. Electronic properties The electronic band structure calculations were performed using HSE06 functional to study the nature of the energy gap and their direct and indirect behavior in ReS2, ReS2-MoS2, and MoS2ReS2 monolayer heterostructure. All direct band gap semiconductor materials including their heterostructure can show novelty to the optoelectronic devices. For the 2D semiconductor materials, the band gap is direct only when their thickness is reduced to the monolayer levels because of quantum confinement occur at this level. Bulk ReS2 is the only material which has a direct band gap in their bulk counterpart as investigated by recent experimental reports and therefore attracts considerable attention.5 However, the situation is now uncertain because even more recent studies of photoluminescence (PL) and reflectivity contrast measurements have reported the observation of an optimal transition in bulk ReS2 having indirect character and 6
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lower energy than the associated to the direct.33 Another report found the value of the band gap is 1.41eV, lower than the 1.5eV direct optimal transition, however, it is in good agreement with the energy of the indirect optimal transition, providing independent confirmations that bulk ReS2 is an indirect band gap semiconductor and have the same property as other two dimensional semiconductor materials (e.g., MoS2, WSe2, etc) have in their bulk counterpart.34 They have reported that ReS2 requires detailed research in relation to feasible optoelectronic applications. Other experimental and theoretical reported results on the optical and electronic properties of ReS2 have been confirmed that ReS2 is a direct band gap semiconductor material.35-37 This contradiction among the researchers on electronic structures of bulk and monolayer ReS2 is still unclear, not only theoretically but also experimentally, this motivates us that it is necessary to clarify the situation. Thus, to shed light and bring closure to these conflicting results, we first present a thorough analysis of the electronic properties of ReS2 using GGA-PBE approximation and further confirm by HSE06. In previous theoretical calculations, they have used GGA-PBE functional which cannot calculate the exact electronic structure of a material, because it always underestimates the band gap value. We also include the Van der Waals interactions in our calculation. In contrast to the previous study, we find that the electronic properties of ReS2 are much more complex than either of the studies they claim. Our analysis on the electronic properties shows that bulk ReS2 have an indirect band gap of 1.4eV. In fact, like the three-dimensional (3D) crystalline materials which exit in hexagonal (wurtzite structure) and in zinc blende (cubic structure), every 2D materials exist in two phases i.e. 2Hphase and 1T-phase but ReS2 can be also crystallized in distorted 1T (Td) phase. In the previous work, the researchers consider mostly 1T phase of ReS2 or distorted 1T with Td symmetry, no attention has been made to the 2H phase of ReS2 which is the solution to all the above conflicting results. The first principle calculation with the semi-local functional GGA-PBE on all the three structures has shown that it is metallic in both 1H-phase and also in 1T phase. Their calculation shows that ReS2 are found to be semiconductor only in distorted 1T (Td) structure. However, the results with 1T phase were recently confirmed by hybrid density functional theory and found that it is a semiconductor with direct band gap material.38 While our calculations are performed in 2H-phase with triclinic symmetry (space group P1) of ReS2 and we also find by the HSE06 calculation that ReS2 is an indirect semiconductor material. This single layer crystal structure is already grown in the experiment.39 Our GGA-PBE and HSE06 results show that ReS2 monolayer 7
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has an indirect band gap of 1.4eV which is a quiet agreement with the experiment both the band gap value and its indirect nature.33, 34, 39 Furthermore, the band structures illustrated in Fig.3 shows that the heterostructures ReS2-MoS2 and MoS2-ReS2 found to be indirect band gap semiconductor materials. Normally, the semi-local functional such as GGA-PBE underestimates the band gaps; thus, the HSE06 Functional is adopted as it already demonstrated that HSE06 functional offers more precise value of band gap in term of experimental observation40. The band structures presented in Fig.3 and Fig.4 represent the indirect band gap nature of MoS2-ReS2 and ReS2-MoS2 monolayer heterostructures, respectively. The reason for the indirect band gap in these two heterostructures is due to the ReS2 which have dominated indirect band gap behavior in the bulk part in the 2H-phase. Our results are also in line with the other theoretical work that type-II band alignment often have an indirect band gap. 4
Energy (eV)
Energy (eV)
2 0
2 0
-2
-2
-4
-4
K
L
(b)
4
(a)
K
L
Fig.2. Band structure of bulk ReS2 with GGA-PBE (a) and HSE06 (b), having indirect band gap. 4
(a)
4
2
Energy (eV)
2 Energy (eV)
(b)
0
0
-2
-2
-4
-4
K
L
L
M
Fig.3. Band structure of ReS2 monolayer with GGA-PBE (a) and HSE06 (b), having indirect band gap.
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(a)
4
Energy (eV)
Energy (eV)
4 2 0 -2
(b)
2 0 -2 -4
-4
L
L
Fig.4. Band structures of ReS2-MoS2 monolayer hetrostructures GGA-PBE (a) HSE06 (b). 4
(a)
2
Energy (eV)
Energy (eV)
4
0 -2
2 0 -2
-4
(b)
-4
L
L
Fig.5. Band structures of MoS2- ReS2 monolayer hetrostructures GGA-PBE (a) HSE06 (b). To understand the physics of different band alignment, localization, and delocalization of electrons and holes, we further investigate the CBM (electrons) and VBM (holes) for both ReS2MoS2 and MoS2-ReS2 monolayer heterostructures. The CBM and VBM are investigated in the term of weighted bands as shown in figure.1. In the first part (top) of Fig. 1, it is clear from the weighted band diagram of ReS2-MoS2 heterostructure that there is a strong contribution of orbitals to the VBM (holes) and no contribution to the CBM (electrons) mentioned i.e. CBM is unaffected. This contribution to the VBM at the L point is due to the d3z 2 r 2 atomic orbital of molybdenum (Mo). In the second part (bottom) the contribution to the VBM is mention and no contribution to the CBM i.e the VBM is contributed by the d x2 y 2 atomic orbital of rhenium (Re). Hence, the contribution of Mo- d3z 2 r 2 and Re- d x2 y 2 orbitals to the VBM and CBM in different layers physically separate electrons and holes pairs. Since such type of contribution to the CBM and VBM from different monolayers gives type-II band alignment nature to the ReS2-MoS2 heterostructure. This is also observed experimentally in such type of ReS2-MoS2 monolayer heterostructures so our findings are in good agreement with the literature20. This Type-II band alignment can make the charge carriers (electrons) to freely move over the heterostructure 9
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interface and hence the heterostructure becomes n-type semiconductor which in turn can be used for the application to the n-type transistors. Recently, different strategies are adopted to bring the type-II band alignment in many other different heterostructure materials. For example, the direct evidence of the type-II band alignment in ZnO-nanorods/Poly(hexylthiophene) heterostructure makes it very useful for the realization of the underpinned mechanism of the developed optoelectronic devices.24 The formation of type-II band alignment in novel heterostructures of functionalizing M-xene makes it a potential candidate for the photocatalytic, photonic and solar energy conversion.41 Similarly, many other novel type-II materials have been reported for different application from optoelectronic to photovoltaic21-25. On the other hand, the VBM at the L-point is contributed by the Mo- d3z 2 r 2 orbital and the CBM at the same symmetry point is also contributed by Mo- d3z 2 r 2 orbital, hence the heterostructures MoS2-ReS2 keeps a type-I band alignment in better agreement with the literature20. In type-I band alignment both CBM and VBM are located in the same material with the narrower band gap. Electrons and holes excited in the wide band gap material transfer to the narrow band gap material in the form of exaction. The quantum confinement of electrons and holes in the same region facilitates their radiative recombination, which is desirable in LEDs application20. As we have already discussed that only six kinds of heterostuctures of the monolayer are investigated and these six kinds of heterostuctures materials have type-II band alignment19. Since the type-I band alignment determination in monolayer heterostructure is a unique property which controls the electrons in the thin monolayers. Such an alignment can provide the hetrostructure a new way to the application of light emitting devices and to study electron-hole pairs (excitons) across the van der Waals interface. Therefore, the researchers using a different technique such as strain engineering and quantum confinement to bring the type-I band alignment to obtain such kind of alignment.
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Fig.6. Weighted band structure of the ReS2-MoS2 monolayer heterostructure Mo (top row), Re (bottom row) for (a) d3z2-r2 (b) d x2 y 2 and (c) dxy 3.2. Charge transfer at the heterostructure interface During the formation of heterostructures sufficient charge transfer occurs from one component to another due to the difference in electronegativity of transition metal atoms (Mo, Re) with chalcogen atoms (S). As sulfur (S) is more electronegative than Mo and Re so they share their electrons with the transition metals to make a strong covalent bond with Re and Mo. In other words, transition metals are more electropositive, so there will be a flow of electrons and holes as well in these semiconductor heterostructures. But for the application to the photocatalysis, there should be an effective spatial separation of electrons and holes. Therefore, for the realization of photocatalyst activity and efficiencies, it is necessary to investigate the charge transfer rate and 11
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localization of electrons and holes in MoS2-ReS2 and ReS2-MoS2 heterostructures. For this purpose, the charge density difference was calculated as Re S2 MoS2 MoS2 Re S 2 , where
Re S 2 MoS 2 is the total charge density of the ReS2-MoS2 heterostructure, MoS2 and Re S2 is the charge density of the pristine MoS2 and ReS2 monolayers respectively. Similarly, for the other heterostructure, the charge density is MoS2 Re S2 MoS2 Re S2 i.e. by subtracting the charge density of the individual monolayers from the charge density of the MoS2-ReS2 heterostructure. Fig.1 shows the spatial charge distribution of MoS2-ReS2 heterostructure. In this heterostructure, the charge density is localized in the ReS2 layer and no charge flow occurs from ReS2 to MoS2 at the interface, this effective spatial separation of charge has potential application to the solar energy harvesting. Secondly, according to an experimental report if there is a lack of charge transfer at the heterostructure then there would be type-I band alignment which has potential application to the optoelectronic application20. Fig.2 shows the charge density difference for the ReS2-MoS2 heterostructure. There is a charge transfer at the interface and the charge are not confined to one layer such type of distribution occurs in type-II band alignment which is necessary for the electronic devices like field effect transistor19. The charge transfer at the interface represents a strong bonding between the two components MoS2 and ReS2 of the heterostructures. The type-II band alignment in ReS2-MoS2 and type-I band alignment in MoS2ReS2 are also reported in the literature19-20.
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Fig.8. The charge transfer rate at the hetrostructure interface for MoS2-ReS2 (left side) and MoS2-ReS2 (right side) monolayer hetrostructures 3.3. Projected Density of States Analysiss (PDOS) The projected density of states of transition elements (Mo, Re) having higher intensities and strong contribution in the conduction band and in the valence band of ReS2-MoS2 and MoS2ReS2 heterostructures. The analysis on PDOS further shows that the transition metals Mo and Re have a dominant contribution to the CBM and VBM as compared to the halogen atoms (S) as shown in Fig.9. In both heterostructures, the transition metals Mo and Re contribute dx2-y2 and dz2-r2 orbitals respectively, to the CBM and VBM and the chalcogen atoms contribute their px and pz sub atomic orbital. This shows the existence of strong p-d hybridization in both heterostructures. Because of the electronegativity difference of chalcogen atoms (S) with the transition metals (Mo, Re) and strong p-d hybridization both heterostructure are considered stable energetically. However, the chalcogen atoms show different behavior to the CBM and VBM, for example in MoS2-ReS2 heterostructres the highest occupied states are going toward the Fermi level while the unoccupied states are going away from the Fermi level while in MoS2ReS2 the occupied states are moving away from the Fermi level and unoccupied states are moving toward the Fermi level. So, in the MoS2-ReS2 heterostructure the upward band bending occurs and in MoS2-ReS2 the downward band bending occurs. Thus, we can conclude that in the band alignments not only the transition metals play a vital role but also the chalcogen atoms. Secondly, in the conduction band of ReS2-MoS2 heterostructure, the most electronegative atom S has a sharp peak with intensity higher than the sharp peak of the S in the conduction band of MoS2-ReS2 heterostructure. This means that the electrons are more confined in the ReS2-MoS2 heterostruture than MoS2-ReS2. This is in line with charge density distribution and also with type-I band alignment.
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Fig.9. PDOS for the d orbital of Mo and Re in ReS2-MoS2 Monolayer heterostructure.
Fig. 10. PDOS for the px, py and pz atomic orbital of chalcogen atoms in ReS2-MoS2 Monolayer heterostructure
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Fig.11. PDOS for the d orbital of Mo and Re in MoS2 -ReS2 Monolayer heterostructure
Fig.12. PDOS for the px, py and pz atomic orbital of chalcogen atoms in MoS2 -ReS2 Monolayer heterostructure 4.
Conclusion
In summary, we investigate the electronic properties of van der Waals heterostructures MoS2ReS2 and ReS2-MoS2 by hybrid density functional theory. We have found that ReS2 is an indirect band gap material in the 2H phase, in agreement with the recent experimental work. Further, the electronic band gap structures show that MoS2-ReS2 heterostructures have type-II band alignment and ReS2-MoS2 shows type-I band alignment with indirect band gaps, in agreement with literature. The charge carriers (electrons and holes) in MoS2-ReS2 heterostructures were localized to the ReS2 layer while shows delocalized character at the interface in ReS2-MoS2 heterostructures. It was further confirmed by charge transfer across at the interface and weighted d bands which contribute to CBM (electrons) and VBM (holes). Acknowledgement This work was financially supported by the grants from the Science and Technology Innovation Commission of Shenzhen (Grant no: JCYJ20180305125302333, ZDSYS201707271554071) and Shenzhen University fund (No. 2017007).
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References [1] Mak KF, Lee C, Hone J, Shan J, Heinz TF. Atomically thin MoS2: a new direct-gap semiconductor. Physical Review Letters 2010;105:136805. [2] McCreary KM, Hanbicki AT, Jernigan GG, Culbertson JC, Jonker BT. Synthesis of largearea WS2 monolayers with exceptional photoluminescence. Scientific reports 2016;6: 19159. [3] Zhang Y, Chang T-R, Zhou B, Cui Y-T, Yan H, Liu Z, et al. Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2. Nature nanotechnology 2014;9:111-5. [4] Sun Q, Dai Y, Ma Y, Jing T, Wei W, Huang B. Ab initio prediction and characterization of Mo2C Monolayer as anodes for lithium-ion and sodium-ion batteries. The journal of physical chemistry letters 2016;7:937-43. [5] Tongay S, Sahin H, Ko C, Luce A, Fan W, Liu K, et al. Monolayer behaviour in bulk ReS2 due to electronic and vibrational decoupling. Nature communications 2014;5: 3252 [6] Samad A, Shafique A, Kim HJ, Shin Y-H. Superionic and electronic conductivity in monolayer W 2 C: ab initio predictions. Journal of Materials Chemistry A 2017;5:11094-11099. [7] Jariwala D, Sangwan VK, Lauhon LJ, Marks TJ, Hersam MC. Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides. ACS nano 2014;8:110220. [8] Yang E, Ji H, Jung Y. Two-dimensional transition metal dichalcogenide monolayers as promising sodium ion battery anodes. The Journal of Physical Chemistry C 2015;119:26374-80. [9] Rasmussen FA, Thygesen KS. Computational 2D materials database: Electronic structure of transition-metal dichalcogenides and oxides. The Journal of Physical Chemistry C 2015;119:13169-83. [10] Ramasubramaniam A. Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides. Physical Review B 2012;86:115409. [11] Peng Q, De S. Outstanding mechanical properties of monolayer MoS2 and its application in elastic energy storage. Physical Chemistry Chemical Physics 2013;15:19427-37. [12] Suk JW, Piner RD, An J, Ruoff RS. Mechanical properties of monolayer graphene oxide. ACS nano 2010;4:6557-64. [13] Zhang K, Feng S, Wang J, Azcatl A, Lu N, Addou R, et al. Manganese doping of monolayer MoS2: the substrate is critical. Nano letters 2015;15:6586-91. 16
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[14] Wang H, Wang Q, Cheng Y, Li K, Yao Y, Zhang Q, et al. Doping monolayer graphene with single atom substitutions. Nano letters 2011;12:141-4. [15] Conley HJ, Wang B, Ziegler JI, Haglund Jr RF, Pantelides ST, Bolotin KI. Bandgap engineering of strained monolayer and bilayer MoS2. Nano letters 2013;13:3626-30. [16] Amin B, Singh N, Schwingenschlögl U. Heterostructures of transition metal dichalcogenides. Physical Review B 2015;92:075439. [17] Fang Q, Huang Y, Miao Y, Xu K, Li Y, Ma F. Interfacial Defect Engineering on Electronic States of Two-Dimensional AlN/MoS2 Heterostructure. The Journal of Physical Chemistry C 2017;121:6605-13. [18] Amin B, Kaloni TP, Schreckenbach G, Freund MS. Materials properties of out-of-plane heterostructures of MoS2-WSe2 and WS2-MoSe2. Applied Physics Letters 2016;108:063105. [19] Zhao M, Zhang W, Liu M, Zou C, Yang K, Yang Y, et al. Interlayer coupling in anisotropic/isotropic van der Waals heterostructures of ReS2 and MoS2 monolayers. Nano Research 2016;9:3772-80. [20] Bellus MZ, Li M, Lane SD, Ceballos F, Cui Q, Zeng XC, et al. Type-I van der Waals heterostructure formed by MoS 2 and ReS2 monolayers. Nanoscale Horizons 2017;2:31-6. [21] Özçelik VO, Azadani JG, Yang C, Koester SJ, Low T. Band alignment of two-dimensional semiconductors for designing heterostructures with momentum space matching. [22] Kang J, Sahin H, Peeters FoM. Tuning carrier confinement in the MoS2/WS2 lateral heterostructure. The Journal of Physical Chemistry C 2015;119:9580-6. [23] Chiu M-H, Zhang C, Shiu H-W, Chuu C-P, Chen C-H, Chang C-YS, et al. Determination of band alignment in the single-layer MoS2/WSe2 heterojunction. Nature communications 2015;6: 7666. [24] Chan M, Chen J, Lin T, Chen Y. Direct evidence of type II band alignment in ZnO nanorods/poly (3-hexylthiophene) heterostructures. Applied Physics Letters 2012;100:021912. [25] Gong C, Zhang H, Wang W, Colombo L, Wallace RM, Cho K. Band alignment of twodimensional transition metal dichalcogenides: Application in tunnel field effect transistors. Applied Physics Letters 2013;103:053513. [26] Lin Y-C, Komsa H-P, Yeh C-H, Bjorkman T, Liang Z-Y, Ho C-H, et al. Single-layer ReS2: two-dimensional semiconductor with tunable in-plane anisotropy. ACS nano 2015;9:11249-57. [27] Xu K, Deng H-X, Wang Z, Huang Y, Wang F, Li S-S, et al. Sulfur vacancy activated field effect transistors based on ReS2 nanosheets. Nanoscale 2015;7:15757-62. 17
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[28] Assali S, Dijkstra A, Li A, Koelling S, Verheijen M, Gagliano L, et al. Growth and optical properties of direct band gap Ge/Ge0.87SN0.13 Core/Shell nanowire arrays. Nano Letters 2017;17:1538-44. [29] Liu E, Fu Y, Wang Y, Feng Y, Liu H, Wan X, et al. Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors. Nature communications 2015;6. [30] Li Y-L, Li Y, Tang C. Strain engineering and photocatalytic application of single-layer ReS 2.
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[31] Wei W, Dai Y, Sun Q, Yin N, Han S, Huang B, et al. Electronic structures of in-plane twodimensional transition-metal dichalcogenide heterostructures. Physical Chemistry Chemical Physics 2015;17:29380-6. [32] Horzum S, Çakır D, Suh J, Tongay S, Huang Y-S, Ho C-H, et al. Formation and stability of point defects in monolayer rhenium disulfide. Physical Review B 2014;89:155433. [33] Gutiérrez-Lezama I, Reddy BA, Ubrig N, Morpurgo AF. Electroluminescence from indirect band gap semiconductor ReS2. 2D Materials 2016;3:045016. [34] Aslan OB, Chenet DA, van der Zande AM, Hone JC, Heinz TF. Linearly polarized excitons in single-and few-layer ReS2 crystals. Acs Photonics 2015;3:96-101. [35] Li X, Cui F, Feng Q, Wang G, Xu X, Wu J, et al. Controlled growth of large-area anisotropic ReS2 atomic layer and its photodetector application. Nanoscale 2016;8:18956-62. [36] He X, Liu F, Hu P, Fu W, Wang X, Zeng Q, et al. Chemical Vapor Deposition of High‐Quality and Atomically Layered ReS2. Small 2015;11:5423-9. [37] Dileep K, Sahu R, Sarkar S, Peter SC, Datta R. Layer specific optical band gap measurement at nanoscale in MoS2 and ReS2 van der Waals compounds by high resolution electron energy loss spectroscopy. Journal of Applied Physics 2016;119:114309. [38] Çakır D, Sahin H, Peeters FM. Doping of rhenium disulfide monolayers: a systematic first principles study. Physical Chemistry Chemical Physics 2014;16:16771-9. [39] Ho C, Liao P, Huang Y, Yang T-R, Tiong K. Optical absorption of ReS2 and ReSe2 single crystals. Journal of applied physics 1997;81:6380-3. [40] Koller D, Blaha P, Tran F. Hybrid functionals for solids with an optimized Hartree–Fock mixing parameter. Journal of Physics: Condensed Matter 2013;25:435503. [41] Lee Y, Hwang Y, Chung Y-C. Achieving type I, II, and III heterojunctions using functionalized MXene. ACS applied materials & interfaces 2015;7:7163-9. 18
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Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Declarations of interest: none
Muhammad Saeed, Waqar Uddin, Awais Siddique Saleemi, Muhammad Hafeez, Madiha Kamil, Irshad Ahmad Mir, Sunila, Rooh Ullah, Shafiq Ur Rehman, Zhu Ling PII:
S0921-4526(19)30705-7
DOI:
https://doi.org/10.1016/j.physb.2019.411809
Reference:
PHYSB 411809
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
05 September 2019
Accepted Date:
21 October 2019
Please cite this article as: Muhammad Saeed, Waqar Uddin, Awais Siddique Saleemi, Muhammad Hafeez, Madiha Kamil, Irshad Ahmad Mir, Sunila, Rooh Ullah, Shafiq Ur Rehman, Zhu Ling, Optoelectronic Properties of MoS2-ReS2 and ReS2-MoS2 Heterostructures, Physica B: Physics of
Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411809
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Optoelectronic Properties of MoS2-ReS2 and ReS2-MoS2 Heterostructures Muhammad Saeed,a,b Waqar Uddin,c Awais Siddique Saleemi,d Muhammad Hafeez,e Madiha Kamil,f Irshad Ahmad Mir,e Sunila,g Rooh Ullah,h Shafiq Ur Rehman*e and Zhu Ling*e aState
Key Laboratory of Nuclear Resources and Enviromement, East China University of Technology, Nanchang, 330013, People, s Republic of China.
bCollege
of Nuclear Science and Engineering, East China University of Technology, Nanchang, 330013, China
cDepartment dInstitute eCollege
of Chemistry, Khushal Khan Khattak University, Kharak, Pakistan
for advanced study, Shenzhen University, Shenzhen, Guangdong 518060, P. R. Chin
of Physics and Optoelectronic Engineering, Shenzhen University, Nanhai Ave. 3688, Shenzhen, Guangdong 518060, People’s Republic of China
fDepartment gDepartment
of Computer Science, Abdul Wali Khan, University, Mardan, 23200, Pakistan
of Physics, Balochistan University of Information Technology, Engineering and Management Sciences, Quetta, Pakistan
hDepartment
of Chemistry, University of Turbat-92600, Balochistan, Pakistan *Corresponding author email: [email protected] *Corresponding author email: [email protected] Abstract
We have investigated the electronic properties of heterostructures MoS2-ReS2 and ReS2-MoS2 with hybrid density functional theory. Contrary to the reported work, we found that ReS2 is an indirect band gap semiconductor material in the 2H phase, in good agreement with experimental work. Furthermore, the calculated charge density profile and weighted bands show that MoS2ReS2 heterostructures have type-II band alignment whereas ReS2-MoS2 have type-I band alignment with indirect band gaps, which are in good agreement with the literature. In MoS2ReS2 heterostructures, both electrons and holes were positioned in ReS2 layer in the form of an exciton, while in ReS2-MoS2 the electrons and holes were located in different layers which
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separated the electrons and holes into two different regions. The former heterostructure material is useful to the photovoltaic devices and the later is for the application of optoelectronic devices. Key words: Band structure engineering, Type-I and Type-II band alignments, ReS2-MoS2 and MoS2-ReS2 monolayers heterostructures, Density functional theory 1. Introduction In the recent decays, the discovery of new class of two-dimensional (2D) semiconductor monolayer materials such as, MoS2, WS2, MoSe2, Mo2C, ReS2, ReSe2, W2C, TiS2, SnS2 and ZrS2, has become a hot topic for the researchers, due to its potential applications in the field of optoelectronic, energy harvesting, gas sensors, catalysis, field effect transistors and storage devices.1-9 These nanoscale materials have astonishing behavior such as tunable electronic energy gap, high optical absorption, and high mechanical strength, which make them superior in the microelectronic industry.10-12 But for the application to the photonic and optoelectronic devices, a proper direct band gap material is essential. Therefore, scientists are searching for new pathways to tune the properties of these semiconductor monolayers such as by doping, strain engineering, staking, and heterostructure to achieve the required band gap material.13-16 By introducing heterostructure between the two materials, the charge carriers (electrons and holes) can be controlled and a desirable optical energy gap material can be achieved.16-18 According to recent experimental reports, only six kinds of MX2/MX2 (MoS2/WS2, MoS2/WSe2, MoS2/MoSe2, WS2/WSe2, MoS2/MoTe2, and MoSe2/WSe2) monolayer heterostructures were fabricated using mechanical transfer techniques or a direct chemical vapor deposition (CVD) method, many future novel devices like field effect transistor (FETs) are based on these heterostructure between the two materials 19-20. On the bases of band alignment, these heterostructures are classified into two types, symmetric (type-I) and staggered (type-II). In type-I band alignment, both the conduction band minimum (CBM) and valence band maximum (VBM) of a narrow energy gap material are located in the wide energy gap material illustrated in Fig. 1(a) & 1(b). Where electrons and holes in the form of excitons are excited in one material and transfer to the other material across the heterostructure interface. These electrons and holes are afterward recombining in one component of the heterostructure which enriches the optical properties of the material. The symmetric Type-I band alignment is most extensively used in certain optical devices e.g., light emitting diodes (LEDs) and in lasers as they provide a way to spatially, 2
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confine electrons and holes so that efficient recombination can occur.20 The type of band alignment where CBM (electrons) and VBM (holes) are not confined to a specific material but are located in different materials is called type-II band alignment. Type-II band alignment is very useful for photovoltaic and photocatalyst devices, since they allow large band offsets on one side.21-25 Rhenium disulfide (ReS2) monolayer is a new member of 2D semiconductors, which played a vital role in industries for the application of photonic devices.26, 27 Since Rhenium disulfide is a new member of transition metal dichalcogenides (TMDs) family, therefore, it exhibits many exotic properties i.e., direct band gap, strong in-plane anisotropy and weak interlayer coupling.5, 27
ReS2 based field effect transistors (FETs) and digital devices showed tremendous electronic
performance. 28, 29 The electronic properties of ReS2 are still a big challenge, it has been reported that it has a direct band gap (1.5eV) in their bulk form and in the monolayer as well5. On the other side, S. Horzum claims with the help of first principle calculations that ReS2 is metal in both 2H and 1T phase.28 While some other theoretical work has investigated that it is indirect semiconductor in bulk and in monolayer as well.30 Since it is important to clarify the misconception among the researchers about their electronic structure before using it in the devices. In the present work, we clarify this inconsistency by using hybrid density functional theory (HSE06). On the other hand, MoS2 is one of the most studied layer semiconductor material and have a variety of application including solar cell, blue light emitting diodes, electroluminescent displays, and photoluminescence devices, etc.1, 10, 15 Moreover, a negligible lattice mismatch was found between the lattice parameter of MoS2 with ReS2 and (mismatch~0.2%) which makes it more attractive for optoelectronic devices in combination with ReS2. Therefore, experimentalists have discovered a very recent ReS2/MoS2 monolayer heterostructure, an indirect band gap semiconductor with type-II band alignment.19. Similarly, an update experimental report on the van der Waals heterostructure (MoS2/ReS2) formed by monolayer’s of MoS2 and ReS2 has been found in the literature, stated that a type-I band alignment exits in such heterostructure with the conduction band minimum and valence band maximum are situated in the ReS2 layer.20 This configuration is unlike from the previous reported type-II van der Waals heterostructures where holes and electrons exist in different layers. The type-I nature of MoS2-ReS2 monolayer heterostructures is confirmed with an experiment by photo-carrier dynamics and transient absorption measurements.20 3
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Motivated by the above experimental work in these heterostrutures, we employ hybrid density functional theory to predict new monolayers ReS2/MoS2 and MoS2/ReS2 heterostructures and their electronic band structure in 2H phase. Here, we demonstrate type-II band alignment and their transition from type-II to type-I band alignment with slightly different intrinsic electric field environment in these heterostructures. Our main focus will be on recent experimental reports, where they have successfully achieved type-II and type-I band alignment in these heterostructures.19,20 We confirm these findings by theoretical investigations and the most important is the type-I alignment, which will be a new prediction by band structure engineering, which enables the heterostructure materials for the highly efficient optoelectronic devices applications. Further, we will also study structure parameters, band structure, localizations of electrons and holes in both ReS2/MoS2 and MoS2/ReS2 heterostructures by using a well-known density functional theory (DFT) approach with GGA-PBE approximation. However, the problem with the semi-local functionals is that it always gives underestimate band gaps values and unclear electronic properties; therefore for further confirmation of our results we employ hybrid density functional HSE06 calculations.21
Fig.1. Description of the Band alignment between two semiconductors a) Type-I band alignment (left side) and b) Type-II alignment (Right side) 2. Methodology Density functional theory, Generalized Gradient Approximation (GGA) with Perdew-BurkeEnzerhof (PBE), calculations was performed in the first step and then their input was used for the HSE06 calculation. Plane Wave pseudopotential code Vienna Ab-initio Simulation Package 4
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(VASP) was used for these simulations. The Van der Waals interactions (Grimme’s approach) were considered to optimize the geometries as well as to perform further calculations. We first built the bulk structures of MoS2, ReS2 separately in its 2H phase than convert it into pristine monolayers. To construct the MoS2-ReS2 monolayer heterostructures, we insert the monolayer of ReS2 into the MoS2 monolayer lattice and for the ReS2-MoS2heterostructure we insert MoS2 in the lattice of ReS2 monolayer. To prevent the interaction between the monolayers a vacuum of 20Å was apply to the c-direction. The structures were relaxed till the forces and energy reached to 0.0001 eV/Å with a high plan wave cutoff of 500 eV. A K-mesh of 12×12×1 was used for the relaxation. 3. Results and discussion The bulk MoS2 was first to design in its 2H phase, as usually 2D transition metal dichalcogenides exist in two phases i.e. 2H-phase and 1T-phase. MoS2 and ReS2 are found to be energetically stable in its 2H phase, therefore, we consider their 2H-phase in the present work. Afterward, we apply a vacuum of 20Å in the c direction to convert this bulk MoS2 into monolayer and the structures were relaxed. The optimized lattice parameters for MoS2 monolayer (a=b= 3.18Å) and the bond length Mo-S is (2.41Å) and for ReS2 monolayer lattice parameters (a=b= 3.16Å), bond length( Re-S=2.40Å) which were in better agreement with literature.31, 32 Further, the ReS2 layer was induced in the MoS2 lattice in such a way that the S atoms of one layer comes on the top of Mo and Re atoms in the other layer and hence the MoS2ReS2 heterostructure were formed with (AA) stacking as shown in Fig.2(a). The lattice parameters for the MoS2-ReS2 are (a=b= 3.16Å), bond lengths (Mo-S= 2.41Å, Re-S= 2.41Å). Where the lattice mismatch (0.2%) between MoS2 and ReS2 is quite small, therefore, there is no need to apply the supercell mechanism to reduce the lattice mismatch. Similarly, the pristine ReS2 monolayer lattice was first created and then insert the MoS2 monolayer in its lattice by introducing ReS2-MoS2 heterostructure shown in Fig. 2(b). For ReS2-MoS2 lattice parameter (a=b= 3.15Å), bond lengths (Mo-S= 2.40Å, Re-S=2.41Å). Further, to understand the relative stability of the ReS2-MoS2 and MoS2-ReS2 heterostructures the binding energy, Eb can be calculated by the difference in the total energy of the heterostructure and their components. The binding energy formula of ReS2-MoS2 and MoS2-ReS2 heterostructures can be defined as
Eb EMoS2 Re S2 EMoS2 ERe S2 5
(1)
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Eb ERe S2 MoS2 ERe S2 EMoS2
(2)
Equation (1) represents MoS2-ReS2 while Equation (2) represents ReS2-MoS2 heterostructures. Where EMoS2 Re S2 and ERe S2 MoS2 represent the total energies of ReS2-MoS2 and MoS2-ReS2 heterostructures per unit cell EMoS2 and ERe S2 are the total energies per unit cell of the individual layers. The calculated binding energies are -0.31, -0.32, -0.34, and -0.37eV for ReS2, MoS2, MoS2-ReS2 and ReS2-MoS2, respectively. Where the negative binding energies of the heterostructures confirm that the two heterostructures are energetically stable. The binding energy of MoS2-ReS2 is lower than ReS2-MoS2 monolayer heterostructures which indicate that the MoS2-ReS2 heterostructure is comparatively more stable than ReS2-MoS2. To check the stability of ReS2-MoS2 and MoS2-ReS2 monolayer heterostructures in the terms of electronic charge distribution, the 3D charge density was calculated by subtracting the electronic charge of ReS2-MoS2 and MoS2-ReS2 heterostructures from their corresponding pristine monolayers i.e. MoS2 and ReS2. Fig.1a shows that there is no charge transfer at the interface of ReS2-MoS2 and hence the heterostructures is less stable because of no strong binding between the two monolayers. On the other hand, the charge density is spread at the interface for MoS2-ReS2 which confirm strong binding between MoS2 and ReS2 and hence MoS2-ReS2 is stable than ReS2-MoS2 hetrostructure. 3.1. Electronic properties The electronic band structure calculations were performed using HSE06 functional to study the nature of the energy gap and their direct and indirect behavior in ReS2, ReS2-MoS2, and MoS2ReS2 monolayer heterostructure. All direct band gap semiconductor materials including their heterostructure can show novelty to the optoelectronic devices. For the 2D semiconductor materials, the band gap is direct only when their thickness is reduced to the monolayer levels because of quantum confinement occur at this level. Bulk ReS2 is the only material which has a direct band gap in their bulk counterpart as investigated by recent experimental reports and therefore attracts considerable attention.5 However, the situation is now uncertain because even more recent studies of photoluminescence (PL) and reflectivity contrast measurements have reported the observation of an optimal transition in bulk ReS2 having indirect character and 6
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lower energy than the associated to the direct.33 Another report found the value of the band gap is 1.41eV, lower than the 1.5eV direct optimal transition, however, it is in good agreement with the energy of the indirect optimal transition, providing independent confirmations that bulk ReS2 is an indirect band gap semiconductor and have the same property as other two dimensional semiconductor materials (e.g., MoS2, WSe2, etc) have in their bulk counterpart.34 They have reported that ReS2 requires detailed research in relation to feasible optoelectronic applications. Other experimental and theoretical reported results on the optical and electronic properties of ReS2 have been confirmed that ReS2 is a direct band gap semiconductor material.35-37 This contradiction among the researchers on electronic structures of bulk and monolayer ReS2 is still unclear, not only theoretically but also experimentally, this motivates us that it is necessary to clarify the situation. Thus, to shed light and bring closure to these conflicting results, we first present a thorough analysis of the electronic properties of ReS2 using GGA-PBE approximation and further confirm by HSE06. In previous theoretical calculations, they have used GGA-PBE functional which cannot calculate the exact electronic structure of a material, because it always underestimates the band gap value. We also include the Van der Waals interactions in our calculation. In contrast to the previous study, we find that the electronic properties of ReS2 are much more complex than either of the studies they claim. Our analysis on the electronic properties shows that bulk ReS2 have an indirect band gap of 1.4eV. In fact, like the three-dimensional (3D) crystalline materials which exit in hexagonal (wurtzite structure) and in zinc blende (cubic structure), every 2D materials exist in two phases i.e. 2Hphase and 1T-phase but ReS2 can be also crystallized in distorted 1T (Td) phase. In the previous work, the researchers consider mostly 1T phase of ReS2 or distorted 1T with Td symmetry, no attention has been made to the 2H phase of ReS2 which is the solution to all the above conflicting results. The first principle calculation with the semi-local functional GGA-PBE on all the three structures has shown that it is metallic in both 1H-phase and also in 1T phase. Their calculation shows that ReS2 are found to be semiconductor only in distorted 1T (Td) structure. However, the results with 1T phase were recently confirmed by hybrid density functional theory and found that it is a semiconductor with direct band gap material.38 While our calculations are performed in 2H-phase with triclinic symmetry (space group P1) of ReS2 and we also find by the HSE06 calculation that ReS2 is an indirect semiconductor material. This single layer crystal structure is already grown in the experiment.39 Our GGA-PBE and HSE06 results show that ReS2 monolayer 7
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has an indirect band gap of 1.4eV which is a quiet agreement with the experiment both the band gap value and its indirect nature.33, 34, 39 Furthermore, the band structures illustrated in Fig.3 shows that the heterostructures ReS2-MoS2 and MoS2-ReS2 found to be indirect band gap semiconductor materials. Normally, the semi-local functional such as GGA-PBE underestimates the band gaps; thus, the HSE06 Functional is adopted as it already demonstrated that HSE06 functional offers more precise value of band gap in term of experimental observation40. The band structures presented in Fig.3 and Fig.4 represent the indirect band gap nature of MoS2-ReS2 and ReS2-MoS2 monolayer heterostructures, respectively. The reason for the indirect band gap in these two heterostructures is due to the ReS2 which have dominated indirect band gap behavior in the bulk part in the 2H-phase. Our results are also in line with the other theoretical work that type-II band alignment often have an indirect band gap. 4
Energy (eV)
Energy (eV)
2 0
2 0
-2
-2
-4
-4
K
L
(b)
4
(a)
K
L
Fig.2. Band structure of bulk ReS2 with GGA-PBE (a) and HSE06 (b), having indirect band gap. 4
(a)
4
2
Energy (eV)
2 Energy (eV)
(b)
0
0
-2
-2
-4
-4
K
L
L
M
Fig.3. Band structure of ReS2 monolayer with GGA-PBE (a) and HSE06 (b), having indirect band gap.
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(a)
4
Energy (eV)
Energy (eV)
4 2 0 -2
(b)
2 0 -2 -4
-4
L
L
Fig.4. Band structures of ReS2-MoS2 monolayer hetrostructures GGA-PBE (a) HSE06 (b). 4
(a)
2
Energy (eV)
Energy (eV)
4
0 -2
2 0 -2
-4
(b)
-4
L
L
Fig.5. Band structures of MoS2- ReS2 monolayer hetrostructures GGA-PBE (a) HSE06 (b). To understand the physics of different band alignment, localization, and delocalization of electrons and holes, we further investigate the CBM (electrons) and VBM (holes) for both ReS2MoS2 and MoS2-ReS2 monolayer heterostructures. The CBM and VBM are investigated in the term of weighted bands as shown in figure.1. In the first part (top) of Fig. 1, it is clear from the weighted band diagram of ReS2-MoS2 heterostructure that there is a strong contribution of orbitals to the VBM (holes) and no contribution to the CBM (electrons) mentioned i.e. CBM is unaffected. This contribution to the VBM at the L point is due to the d3z 2 r 2 atomic orbital of molybdenum (Mo). In the second part (bottom) the contribution to the VBM is mention and no contribution to the CBM i.e the VBM is contributed by the d x2 y 2 atomic orbital of rhenium (Re). Hence, the contribution of Mo- d3z 2 r 2 and Re- d x2 y 2 orbitals to the VBM and CBM in different layers physically separate electrons and holes pairs. Since such type of contribution to the CBM and VBM from different monolayers gives type-II band alignment nature to the ReS2-MoS2 heterostructure. This is also observed experimentally in such type of ReS2-MoS2 monolayer heterostructures so our findings are in good agreement with the literature20. This Type-II band alignment can make the charge carriers (electrons) to freely move over the heterostructure 9
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interface and hence the heterostructure becomes n-type semiconductor which in turn can be used for the application to the n-type transistors. Recently, different strategies are adopted to bring the type-II band alignment in many other different heterostructure materials. For example, the direct evidence of the type-II band alignment in ZnO-nanorods/Poly(hexylthiophene) heterostructure makes it very useful for the realization of the underpinned mechanism of the developed optoelectronic devices.24 The formation of type-II band alignment in novel heterostructures of functionalizing M-xene makes it a potential candidate for the photocatalytic, photonic and solar energy conversion.41 Similarly, many other novel type-II materials have been reported for different application from optoelectronic to photovoltaic21-25. On the other hand, the VBM at the L-point is contributed by the Mo- d3z 2 r 2 orbital and the CBM at the same symmetry point is also contributed by Mo- d3z 2 r 2 orbital, hence the heterostructures MoS2-ReS2 keeps a type-I band alignment in better agreement with the literature20. In type-I band alignment both CBM and VBM are located in the same material with the narrower band gap. Electrons and holes excited in the wide band gap material transfer to the narrow band gap material in the form of exaction. The quantum confinement of electrons and holes in the same region facilitates their radiative recombination, which is desirable in LEDs application20. As we have already discussed that only six kinds of heterostuctures of the monolayer are investigated and these six kinds of heterostuctures materials have type-II band alignment19. Since the type-I band alignment determination in monolayer heterostructure is a unique property which controls the electrons in the thin monolayers. Such an alignment can provide the hetrostructure a new way to the application of light emitting devices and to study electron-hole pairs (excitons) across the van der Waals interface. Therefore, the researchers using a different technique such as strain engineering and quantum confinement to bring the type-I band alignment to obtain such kind of alignment.
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Fig.6. Weighted band structure of the ReS2-MoS2 monolayer heterostructure Mo (top row), Re (bottom row) for (a) d3z2-r2 (b) d x2 y 2 and (c) dxy 3.2. Charge transfer at the heterostructure interface During the formation of heterostructures sufficient charge transfer occurs from one component to another due to the difference in electronegativity of transition metal atoms (Mo, Re) with chalcogen atoms (S). As sulfur (S) is more electronegative than Mo and Re so they share their electrons with the transition metals to make a strong covalent bond with Re and Mo. In other words, transition metals are more electropositive, so there will be a flow of electrons and holes as well in these semiconductor heterostructures. But for the application to the photocatalysis, there should be an effective spatial separation of electrons and holes. Therefore, for the realization of photocatalyst activity and efficiencies, it is necessary to investigate the charge transfer rate and 11
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localization of electrons and holes in MoS2-ReS2 and ReS2-MoS2 heterostructures. For this purpose, the charge density difference was calculated as Re S2 MoS2 MoS2 Re S 2 , where
Re S 2 MoS 2 is the total charge density of the ReS2-MoS2 heterostructure, MoS2 and Re S2 is the charge density of the pristine MoS2 and ReS2 monolayers respectively. Similarly, for the other heterostructure, the charge density is MoS2 Re S2 MoS2 Re S2 i.e. by subtracting the charge density of the individual monolayers from the charge density of the MoS2-ReS2 heterostructure. Fig.1 shows the spatial charge distribution of MoS2-ReS2 heterostructure. In this heterostructure, the charge density is localized in the ReS2 layer and no charge flow occurs from ReS2 to MoS2 at the interface, this effective spatial separation of charge has potential application to the solar energy harvesting. Secondly, according to an experimental report if there is a lack of charge transfer at the heterostructure then there would be type-I band alignment which has potential application to the optoelectronic application20. Fig.2 shows the charge density difference for the ReS2-MoS2 heterostructure. There is a charge transfer at the interface and the charge are not confined to one layer such type of distribution occurs in type-II band alignment which is necessary for the electronic devices like field effect transistor19. The charge transfer at the interface represents a strong bonding between the two components MoS2 and ReS2 of the heterostructures. The type-II band alignment in ReS2-MoS2 and type-I band alignment in MoS2ReS2 are also reported in the literature19-20.
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Fig.8. The charge transfer rate at the hetrostructure interface for MoS2-ReS2 (left side) and MoS2-ReS2 (right side) monolayer hetrostructures 3.3. Projected Density of States Analysiss (PDOS) The projected density of states of transition elements (Mo, Re) having higher intensities and strong contribution in the conduction band and in the valence band of ReS2-MoS2 and MoS2ReS2 heterostructures. The analysis on PDOS further shows that the transition metals Mo and Re have a dominant contribution to the CBM and VBM as compared to the halogen atoms (S) as shown in Fig.9. In both heterostructures, the transition metals Mo and Re contribute dx2-y2 and dz2-r2 orbitals respectively, to the CBM and VBM and the chalcogen atoms contribute their px and pz sub atomic orbital. This shows the existence of strong p-d hybridization in both heterostructures. Because of the electronegativity difference of chalcogen atoms (S) with the transition metals (Mo, Re) and strong p-d hybridization both heterostructure are considered stable energetically. However, the chalcogen atoms show different behavior to the CBM and VBM, for example in MoS2-ReS2 heterostructres the highest occupied states are going toward the Fermi level while the unoccupied states are going away from the Fermi level while in MoS2ReS2 the occupied states are moving away from the Fermi level and unoccupied states are moving toward the Fermi level. So, in the MoS2-ReS2 heterostructure the upward band bending occurs and in MoS2-ReS2 the downward band bending occurs. Thus, we can conclude that in the band alignments not only the transition metals play a vital role but also the chalcogen atoms. Secondly, in the conduction band of ReS2-MoS2 heterostructure, the most electronegative atom S has a sharp peak with intensity higher than the sharp peak of the S in the conduction band of MoS2-ReS2 heterostructure. This means that the electrons are more confined in the ReS2-MoS2 heterostruture than MoS2-ReS2. This is in line with charge density distribution and also with type-I band alignment.
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Fig.9. PDOS for the d orbital of Mo and Re in ReS2-MoS2 Monolayer heterostructure.
Fig. 10. PDOS for the px, py and pz atomic orbital of chalcogen atoms in ReS2-MoS2 Monolayer heterostructure
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Fig.11. PDOS for the d orbital of Mo and Re in MoS2 -ReS2 Monolayer heterostructure
Fig.12. PDOS for the px, py and pz atomic orbital of chalcogen atoms in MoS2 -ReS2 Monolayer heterostructure 4.
Conclusion
In summary, we investigate the electronic properties of van der Waals heterostructures MoS2ReS2 and ReS2-MoS2 by hybrid density functional theory. We have found that ReS2 is an indirect band gap material in the 2H phase, in agreement with the recent experimental work. Further, the electronic band gap structures show that MoS2-ReS2 heterostructures have type-II band alignment and ReS2-MoS2 shows type-I band alignment with indirect band gaps, in agreement with literature. The charge carriers (electrons and holes) in MoS2-ReS2 heterostructures were localized to the ReS2 layer while shows delocalized character at the interface in ReS2-MoS2 heterostructures. It was further confirmed by charge transfer across at the interface and weighted d bands which contribute to CBM (electrons) and VBM (holes). Acknowledgement This work was financially supported by the grants from the Science and Technology Innovation Commission of Shenzhen (Grant no: JCYJ20180305125302333, ZDSYS201707271554071) and Shenzhen University fund (No. 2017007).
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Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Declarations of interest: none