Superlattices and Microstructures 151 (2021) 106841
Available online 15 February 2021
Effects of La and Ce doping on electronic structure and optical properties of janus MoSSe monolayer
Thi-Nga Do
a,b, C.V. Nguyen
c, Lam V. Tan
d, M. Idrees
e, Bin Amin
f, Nguyen V. Hieu
g, Nguyen T.X. Hoai
g, Le T. Hoa
h,i, Nguyen N. Hieu
h,i,*, Huynh V. Phuc
jaLaboratory of Magnetism and Magnetic Materials, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
bFaculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
cDepartment of Materials Science and Engineering, Le Quy Don Technical University, Ha Noi, Viet Nam
dNTT Hi-Tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City, Viet Nam
eDepartment of Physics, Hazara University, Mansehra, 21300, Pakistan
fDepartment of Physics, Abbottabad University of Science and Technology, Abbottabad, 22010, Pakistan
gDepartment of Physics, The University of Danang, University of Science and Education, Da Nang, Viet Nam
hInstitute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam
iFaculty of Natural Sciences, Duy Tan University, Da Nang, 550000, Viet Nam
jDivision of Theoretical Physics, Dong Thap University, Dong Thap, Viet Nam
A R T I C L E I N F O Keywords:
Janus MoSSe monolayer Doping
Rare earth metals DFT calculations
A B S T R A C T
In this work, the doping effects of rare-earth La and Ce atoms on electronic and optical properties of Janus MoSSe monolayer are investigated by means of first principles calculations. Our results imply that when one La and Ce doped to one S or Se side of Janus MoSSe monoalayer, it leads to a decrease in the band gap and results in the transition from direct to indirect. With increasing the La and Ce doping concentration, the Janus MoSSe monolayer switched from semiconductor to metal. Moreover, we find that effective masses of all the La and Ce doped Janus MoSSe systems are decreased as compared to pristine state, rendering their high carrier mobility. Furthermore, all the La and Ce doped MoSSe systems have red shift and possess high absorption ability in the visible and infrared regions. These findings suggest that rare-earth La and Ce doped MoSSe monolayer are potential candidate for spintronics, nanoelectronics and optoelectronics.
1. Introduction
Following the development of materials science, a lot of new materials with appropriated physical and chemical properties that are required for the design of high-performance nanodevices have been predicted and synthesized. The discovery of graphene [1] in 2004 by Geim and his group has opened up a new chapter for thin film two-dimensional (2D) materials, which can be considered as promising candidate for various applications, including optoelectronic, photocatalyst, gas sensors, light-emitting diodes [2–5]. To now, there are many 2D materials, including hexagonal boron nitride (hBN) [6,7], phosphorene [8,9], transition metal dichalcoge- nides (TMDCs) [10–13] and graphitic carbon nitrides [14,15]. However, the most of 2D materials in their freestanding monolayers
* Corresponding author. Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam.
E-mail addresses: dothinga@tdtu.edu.vn (T.-N. Do), hieunn@duytan.edu.vn (N.N. Hieu).
Contents lists available at ScienceDirect
Superlattices and Microstructures
journal homepage: www.elsevier.com/locate/superlattices
https://doi.org/10.1016/j.spmi.2021.106841
Received 23 April 2020; Received in revised form 1 August 2020; Accepted 8 February 2021
Superlattices and Microstructures 151 (2021) 106841
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have a drawback that may hinder their applications in high-efficient devices. For instance, the lack of an electronic band gap in graphene makes it incompatible with high-speed logic circuit devices [16]. Molybdenum disulfide (MoS2) is one of the most famous TMDCs, it exhibits a semiconducting nature with a suitable band gap of about 2 eV for semiconductor energy devices [17]. Unfor- tunately, due to a small carrier mobility of about 200 cm2/V, MoS2 monolayer is incompatible with high-performance nanoelectronic devices [18]. Therefore, along with the design and synthesis of new 2D materials, one of the most important tasks for research on 2D materials is how to tune the properties and extend the range of applications of these 2D materials.
More recently, a new type of the TMDCs, namely Janus MoSSe, has been successfully synthesized in recent experiments [19,20]. It is interesting that Janus MoSSe was obtained from MoS2 monolayer by replacing one sulfide layer with selenium layer. Thus, it results in the formation of the asymmetric structure of Janus MoSSe. Thank to this asymmetric structure, Janus MoSSe has many advantages as compared with TMDCs, such as Rashba spin splitting [21], piezoelectric effect [22]. Moreover, Janus MoSSe is known to be dynamically stable at room temperature [23]. Nowadays, many strategies can be used to modulate the interesting properties of Janus MoSSe monolayer, such as strain [24], layer stacking [25], electric field [21], doping [26,27]. The tunable electronic, magnetic and transport properties of Janus MoSSe make it promising potential for various applications of spintronics, nanoelectronics and optoelectronics.
Recently, the impurity doping has been proved to be one of the most strategies to modify the physico-optical properties of graphene and the formation of novel graphene-like 2D materials [28–32]. For example, Hashmi et al. [28] studied theoretically the transition metals doped phosphorene using first-principles calculations. They demonstrated that the doping of the transition metals results in the formation of spin polarized states in phosphorene, making it potential material for spintronic applications. Andriotis [31] considered the effect of transition metal doped into Mo2 monolayer using ab initio simulation. Moreover, the effect of co-doping on the MoS2 by cation-cation and cation-anion pairs is also examined. The results indicated that the transition metal doping of MoS2 monolayer tends to decrease in the band gap and to an appearance of magnetic behavior of MoS2 material. Furthermore, the effect of transition metal doping, including Cr, Mn, Fe, Co, and Ni metals on electronic and magnetic properties of Janus monolayers has also been investigated [27,33]. Recently, several methods that have proved that it is possible to introduce dopants on anionic sites of nonmagnetic two-dimensional materials, such as pulsed laser deposition, and defect-assisted doping by electron beam irradiation [34,35]. Theo- retically, Fuhr et al. [29] studied the substitution of a S atom in MoS2 surface by metal (Pd, Au, Fe and V) atoms. Andriotis [31]
investigated the effect of co-doping on the MoS2 by combining the cation-cation and cation-anion pairs. To our best knowledge, there is no research result about rare earth atoms (RE) doped Janus MoSSe monolayer. We therefore provide an accurate description of the electronic structure in terms of first principle calculations. A comprehensive insight is gained to explore the electronic properties, charge distributions and optical properties of RE-doped Janus MoSSe monolayer. We find that both the La and Ce doped result in a decrease in the band gap and the transition from semiconductor to metal is emerged with both La and Ce codoped to both side of MoSSe. In addition, Janus MoSSe monolayer with the La and Ce doped has a red shift of the absorption peaks and high absorption ability in the visible and infrared region.
2. Computational details
In this work, density functional theory from first principle calculations that implemented by Quantum Espresso [36] under plane wave (PW) basis set [37] and pseudopotentials (PP) [38] is used. The exchange correlation functional is elucidated by the virtue of Perdew-Burke-Ernserhof (PBE) functional [39] with the contrivance of generalized gradient approximation (GGA) [40]. The plane wave augmentation is achieved when the energy cutoff is stipulated to 500 eV. The Monkhorst-Pack kpoint grid is established as 12× 12×1 which corresponds to first Brillouin zone. The optimized atomic positions are obtained when the threshold for energy (forces) is achieved to 10−4 eV (eV/Å). All the entities are provided with a vacuum level of 25 Å along z-direction in order to avoid factitious interactions between the layers of atoms. In order to investigate the effects of doping, we used a large supercell of 4 × 4 × 1 of Janus MoSSe monolayer, which contains of 16 Mo, 16 S and 16 Se atoms. The doping process was done by replacing S and Se atoms by La and Ce atoms, which yielded the doping concentration of 2.1%, 4.2% and 8.4%.
3. Results and discussions
We first check the geometric structure of Janus MoSSe monolayer, which was fully optimized with the lattice constant of 3.25 Å that Table 1
Calculated bond lengths (Å), band gap (eV), lattice parameters (Å), and effective masses (m*/m0) of one La and Ce doped Janus MoSSe monolayer, respectively.
Pristine MoSSe La-doped Ce-doped La-doped Ce-doped
S side Se side
Mo–S 2.42 2.40 2.40
Mo–Se 2.53 2.45 2.45
Eg 2.24 0.45 0.56 0.32 0.40
A 3.25 3.25 3.25
μh 0.62 0.52 0.56 0.54 0.59
μe 0.52 0.44 0.50 0.46 0.49
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Γ Κ Μ Γ
0 100 200 300 400 500
(cm-1 )
Γ Κ Μ Γ Κ
-3 -2 -1 0 1 2 3
Energy (eV)
0 2 4 6 8 10
Energy (eV)
0 5 10 15 20
ε2(ω)
ω
(a)
(b) (c) (d)
Fig. 1.(a) The atomic structure (b) Phonon band structure, (c) Electronic band structure and (d) imaginary part of dielectric function of MoSSe monolayer. Cyan, blue and yellow circles represent the Mo, S and Se atoms, respectively.
Do et al.
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given from our previous report [41]. We find that the Mo–S and Mo–Se bond lengths of Janus MoSSe monolayer are 2.42 Å and 2.53 Å, respectively. All these results are also listed in Table 1. The atomic structure of MoSSe monolayer is depicted in Fig. 1. One can find that in the Janus MoSSe monolayer, the Mo atoms are sandwiched between two different chalcogens S and Se atoms. Due to different kinds of chalcogen atoms in both sides of monolayer, it results in the breaking of out of plane symmetry.
We further check the dynamical stability of Janus MoSSe monolayer by performing the phonon spectrum calculations, as illustrated in Fig. 1 (b). We find that Janus MoSSe monolayer is dynamically stable with no soft modes in its phonon spectrum. Phonon spectrum of Janus MoSSe monolayer consists three acoustics, including in-plane longitudinal, transverse and vertical acoustics, and six optical branches. of two in-plane longitudinal optical, two in-plane transverse optical, and two out-of-plane optical modes. Six optical modes are further classed as degenerate and non degenerate. Vibration frequencies of degenerated and non-degenerated modes are in agreement with the Raman peaks at 287 and 355 cm−1 of MoS2 monolayer [19]. The band structure and imaginary part of dielectric functions of Janus MoSSe monolayer are also examined and depicted in Fig. 1(c and d). The band structure of MoSSe shows that it is direct band nature with both the CBM and VBM at K point, as illustrated in Fig. 1(c). The band gap of MoSSe is calculated to be 2.24 eV using HSE06 approach. The imaginary part of dielectric function shows that the A and B-excitons of Janus MoSSe monolayer are in the energy range from 2.0 eV to 2.2 eV.
We now consider the effect of doping La and Ce atoms into Janus MoSSe monolayer with 4 ×4 supercell. The La and Ce atoms are
Γ Κ Μ Γ
-1 0 1
Energy (eV)
Γ Κ Μ ΓΓ Κ Μ ΓΓ Κ Μ ΓΓ Κ Μ ΓΓ Κ Μ Γ
Fig. 2.Doping of La atoms in MoSSe monolayer, and the band structure of doped La–MoSSe layers, the red color is for La atoms which gradually increases from going left to right.
Γ Κ Μ Γ
-1 0 1
Energy (eV)
Γ Κ Μ ΓΓ Κ Μ ΓΓ Κ Μ ΓΓ Κ Μ ΓΓ Κ Μ Γ
Fig. 3.Doping of Ce atoms in MoSSe monolayer, and the band structure of doped Ce–MoSSe layers, blue color is for Ce atoms which gradually increases from going left to right.
T.-N. Do et al.
doped into MoSSe monolayer by replacing the positions of S or Se atoms or both of them. The atomic structures of La-doped MoSSe monolayer in S, Se and both S, Se sides are illustrated in top panel of Fig. 2. We find that one La (La1) doped into S side of Janus MoSSe monolayer for model 1′′in Fig. 2 tends to decrease in the Mo–S and Mo–Se bond lengths to 2.40 Å and 2.45 Å, respectively. The La1 doped MoSSe monolayer possesses indirect band gap semiconductor, indicating the switchable from direct to indirect transition of band nature. However the La1-doped S side leads to a decrease in the band gap of MoSSe monolayer from 2.24 eV to 0.45 eV. Similar to La1 doped S side in model 1′′, La1 doped Se side of Janus MoSSe in model 2” also tends to decrease in the bond lengths and band gap value of MoSSe monolayer. The band gap of La1 doped Se side is calculated to be 0.32 eV, which is larger than that of La1 doped S side.
It means that one La1 doped Se side shows strong effect than one La1 doped S side of Janus MoSSe monolayer. When two La atoms (La2) are doped on S side (La2S) and Se side (La2Se), it is interesting that the semiconducting nature of MoSSe monolayer is converted into metallic one. If we replaced at a time one S and one Se by La atom, it also shows metallic nature. And when we replaced two S and two Se atoms by four La atoms, the semiconductor nature of MoSSe is also switched into metallic one.
The atomic structures and band structures of Ce doped MoSSe monolayer are displayed in Fig. 3. One can find from Fig. 3 that the Ce doped MoSSe monolayer has similar changing trends of the electronic properties as compared with La doped. When one Ce doped MoSSe monolayer either for S or Se side, it tends to the transition from direct to indirect semiconductor and leads to a decrease in the band gap of Janus MoSSe monolayer. The band gap of MoSSe monolayer decreases from 2.24 eV in perfect form to 0.56 eV in Ce1S and to 0.40 eV in Ce1Se doped. The transformation from semiconductor to metal is observed in MoSSe monolayer when two Ce doped to both sides of S and Se atoms.
The partial density of state (PDOS) of La doped MoSSe monolayer are depicted in Fig. 4 with the contributions of all sub-orbitals of all S-p, Se-p, Mo (dxy, dxz, dyz, dz2 and dx2) and La (dxy, dxz, dyz, dz2 and dx2) atoms. When La1 doped MoSSe monolayer to form La1S and La1Se, the doped MoSSe monolayer possesses semiconducting nature. The VBM of La1S doped MoSSe is contributed mainly by Se atoms, whereas its CBM comes from sub-orbitals dxy and dz2 of Mo atoms, as illustrated in Fig. 4(a). Similarly, the VBM of La1Se doped MoSSe monolayer is mainly contributed by S atoms, while its CBM is due to the sub-orbitals dxy and dz2 of Mo atoms, as depicted in Fig. 4(b). With increasing the concentration of La doped MoSSe monolayer, one can find that both the CBM and VBM of La doped MoSSe monolayer cross the Fermi level, resulting in the formation of metallic character, as displayed in Fig. 4(c–f). Thus, we can conclude that the La doped MoSSe monolayer leads to the transition from semiconductor to metal.
Furthermore, the PDOSs of the Ce doped MoSSe monolayer for all sub-orbitals of S-p, Se-p, Mo (dxy, dxz, dyz, dz2 and dx2) and Ce (dxy, dxz, dyz, dz2 and dx2) atoms are depicted in Fig. 5. In the case of Ce1S as depicted in Fig. 5(a), one Ce doped MoSSe monolayer exhibits semiconductor nature. The CBM is mainly contributed by the p-states of S and Se layers, whereas the VBM comes from Mo-dxy sub- orbital. In the case of Ce1Se as depicted in Fig. 5(b), the Ce doped MoSSe monolayer keeps the semiconducting nature. However, the CBM of the Ce1Se structure comes from p-state of Se layer, whereas the VBM is due to the Mo-dyz sub-orbital. With increasing Ce Fig. 4.Partial Density of State (PDOS) of La-doped in MoSSe monolayer for (a) La1S, (b) La1Se, (c) La2S, (d) La2Se, (e) La1SSe and (f) La2SSe, respectively.
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Fig. 5.Partial Density of State (PDOS) of Ce-doped in MoSSe monolayer for Ce1S, (b) Ce1Se, (c) Ce2S, (d) Ce2Se, (e) Ce1SSe and (f) Ce2SSe, respectively.
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doping concentration, the transition from semiconductor to metal is appeared. Both the CBM and VBM of Ce doped MoSSe monolayer cross the Fermi level, as depicted in Fig. 5(c–f).
Furthermore, we calculate the charge density difference in all cases of Ce doped MoSSe monolayer as follows: Δρ =ρCe−MoSSe - ρMoSSe - ρnCe, where ρCe−MoSSe, ρMoSSe, and ρnCe represent the charge densities of Ce doped MoSSe, isolated MoSSe and Ce atom. n is the number of Ce doped atoms. The yellow color shows the electron-rich regions, where red color denotes the hole-rich region. It indicates that most of the charges are depleted from the metal atoms, although each atom has a different trends, as illustrated in Fig. 6.
A sophisticated measurement of the effective mass of the charge carriers is in contrivance with the Deformation Potential Theory offering expectation value of electron-phonon interaction potential. The effective masses of the carriers are evaluated by the equation;
m* =ℏ2 (∂2∂kE(2k))−1 We have investigated the effective masses of electrons in terms of their rest mass. The results reveal that the effective masses of simple MoSSe monolayer and La (Ce) doped on S and Se side is given in Tab 1. It is clear that the small effective mass leads to high carrier mobility and the comparative study divulges that La doped on S side possess relatively low consequently renders high carrier mobility. Such a material offers its applications in high electron mobility transistor (HETM) devices.
The optical absorption behaviors in term of imaginary part of dielectric function of La and Ce doped in MoSSe are presented in Fig. 7 and Fig. 8. It shows absorption peaks in visible light region and several peaks at the ultraviolet region. The lowest energy transitions are dominated by excitons. To compare these results with pristine MoSSe monolayer, one can observe that all the La and Ce Fig. 6.Charge density difference of Ce doped MoSSe monolayer. Yellow and red areas represent the charge accumulation and depletion, respectively.
Fig. 7.Imaginary part of La-doped in MoSSe monolayer as a function of photon energy.
Superlattices and Microstructures 151 (2021) 106841
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doped shows a red shift. The similar effects have also observed and explained in our previous works [42]. Moreover, one can find from the imaginary part of La and Ce doped in MoSSe that they show red shift in the absorption peaks and modulate the optical propitiates of MoSSe monolayer. These doped systems shows red shift as compared to the simple MoSSe monolayer. The red or blue shift, in fact, is a quantum phenomenon and describes that when light comes in contact with the material, it either gains or loses some quanta by interacting with the vibrational modes (phonons) of the material. Whenever higher energy absorption takes place due to exciton, it leads to a shift towards smaller wavelength (high energy) blue shift, and when energy is lost, a red shift is observed. These results demonstrate that the optical properties of Janus MoSSe monolayer can be improved by La and Ce doping.
4. Conclusion
In summary we have investigated the optoelectronic properties of La and Ce doped Janus MoSSe monolayer by using density functional theory. By doping only one La and one Ce atom on either side of S or Se reduced the band gap of MoSSe monolayer while increasing the doping of these rare earth elements the MoSSe change to metallic one. Imaginary part shows that the La and Ce doped in MoSSe exhibA-t red shift in the absorption peaks and modulate the optical propitiates of MoSSe monolayer. The peaks are observed in ˜ visible and infrared region. Our calculation indicate that La and Ce doped in MoSSe monolayer improve the optoelectronic properties and shows potential application in spintronic and optoelectronic devices.
Author contributions
Thi-Nga Do: Software, Investigation, Validation, Writing – original draft. Chuong V. Nguyen: Conceptualization, Supervision, Writing – original draft, Writing – review & editing, Funding acquisition. Lam V. Tan: Software, Investigation, Validation. M. Idrees:
Methodology, Software, Investigation. Bin Amin: Investigation, Validation. Nguyen V. Hieu: Methodology, Software, Investigation.
Nguyen T. X. Hoai: Methodology, Software, Investigation. Le T. Hoa: Investigation, Validation. Nguyen N. Hieu: Software, Investi- gation. Huynh V. Phuc: Methodology, Investigation, Validation.
Declaration of competing interest
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.
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