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C60 van der Waals heterostructures: A first-principles study
Journal Pre-proofs Full Length Article Investigation of the structural and electronic properties of pristine and Au-embedded MoS2/C60 and WSe2/C60 van der Waals heterostructures: A firstprinciples study Xiaohui Lu, Mingxuan Cui, Xicai Pan, Peifang Wang, Lingjie Sun PII: DOI: Reference:
S0169-4332(19)33144-7 https://doi.org/10.1016/j.apsusc.2019.144328 APSUSC 144328
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Please cite this article as: X. Lu, M. Cui, X. Pan, P. Wang, L. Sun, Investigation of the structural and electronic properties of pristine and Au-embedded MoS2/C60 and WSe2/C60 van der Waals heterostructures: A firstprinciples study, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144328
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Investigation of the structural and electronic properties of pristine and Au-embedded MoS2/C60 and WSe2/C60 van der Waals heterostructures: A first-principles study
Xiaohui Lu a*, Mingxuan Cui a, Xicai Pan b, Peifang Wang a, Lingjie Sun c
a. School of Earth Science and Engineering, Ministry of Education, Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Hohai University, Nanjing, 210098, China b. Institute of Soil Sciences, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing, China c. School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, 410004, China
* Corresponding Author: [email protected] (X. Lu)
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Abstract The interaction of C60 molecule with pristine and Au-decorated MoS2 and WSe2 nanosheets were investigated using the density functional theory calculations. The results suggest that adsorption of C60 molecule on the surface of Au-embedded MoS2 is more energetically favorable than that on the pristine MoS2 sheet. Thus, Au-embedded MoS2 systems can interact with C60 molecule more strongly. The interaction of C60 molecule with WSe2 nanosheet was also found to be an energy favorable adsorption process. The large overlaps between the projected density of states of the carbon and Ag/Au atoms indicate the covalent nature of the interaction between them, manifested by the newly formed chemical bonds at the interface. This formation of covalent bonds was also revealed by the charge density difference plots of the considered systems. There is some charge accumulation between the newly formed Au-C bonds, indicating the covalent characteristics of the interaction. Based on the band structure plots, we found that the Agembedded and Au-embedded MoS2/C60 heterostructures show metallic behavior, whereas the pristine heterostructure exhibit semiconductor characteristics.
Keywords: Density functional theory; Ag-embedded MoS2/C60; Au-embedded MoS2/C60; WSe2/C60; Band structure
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1. Introduction New advances in materials science and engineering have been attained by proposing novel 2D materials that are constructed from the various kinds of structures, which leads to some unique electronic, magnetic and optical properties. By minimizing the size of devices, the optical and electronic properties of low-dimensional materials can be effectively modulated. Research in this field has aroused a large number of attention and becomes a hot topic for experimental and theoretical investigations [1-3]. After the successful fabrication of graphene in the laboratory, which has unique physical properties as a 2D material, lots of investigations have been devoted to development of graphene-like 2D layered nanomaterials and heterostructure systems composed of these 2D monolayers. In these heterostructures, there are strong covalent bonds between the atoms of each monolayer, while the interlayer interaction between these monolayers is described based on weak van der Waals (vdW) forces [4-6]. Furthermore, these 2D heterostructures may display unique electronic properties compared with the bulk counterparts such as semiconductor/metal phase transitions, direct/indirect band gap transitions, and so on. Because of these outstanding properties, 2D layered materials are of great importance to construct vdW heterojunctions with extensive applications in the next generation nanoelectronic and optoelectronic devices [7-9]. In the past few years, carbonaceous materials such as graphene, fullerene, and carbon nanotubes have attracted significant interests owing to their special structures and extraordinary electronic properties [10, 11], which lead to the great improvements on the photocatalytic performances of semiconductors. For example, the combination of C3N4 with carbonaceous materials gives rise to a novel heterojunction photocatalysts with unique properties. For instance, heterostructures composed of graphene and C3N4, fabricated by the impregnation–chemical reduction strategy, exhibit great visible light photocatalytic activity for some processes such as hydrogen production 3
[12]. Recent investigations have revealed that composite systems containing graphene and many two-dimensional materials greatly enhance the electronic and optoelectronic properties of 2D materials [13-15]. Hence, we aim at studying the electronic and structural properties of molybdenum disulfide (MoS2) embedded with C60 molecules because of its unique band structures and consequently satisfied band gap. To the best of our knowledge, no theoretical study has been performed on the exploration of the electronic, magnetic and structural properties of MoS2/C60 hybrid nanosystems. However, there exists some investigations on the interaction of different 2D nanomaterials with external agents (gas molecules/transitional metals) focusing on their applicability in gas sensor devices [16-20]. MoS2/graphene heterostructures and graphene-like MoS2/amorphous carbon hybrid systems have been fabricated in the laboratory by Chang et al. [21-22]. The first principles calculations indicate that C60-interfaced TiO2 in both the covalent linking and mechanical mixture cannot form a promising photovoltaic heterojunction [23], while B- or N doped C60/MoS2 or C60/WS2 heterostructures can form an effective photovoltaic heterojunction [24]. The novel electronic and structural properties of different heterostructures consisting of transition metal dichalcogenides and other 2D materials have been systematically explored [25-30]. For instance, Abbasi et al. [3133] have examined the gas sensing properties of heterostructures composed of TiO2 and WSe2/Stanene monolayers. In this work, we carried out a density functional theory (DFT) study to explore the structural and electronic properties of a van der Waals heterostructure constructed from C60 fullerene and MoS2 or WSe2 monolayers. The interaction of C60 fullerene with both pristine and Au-embedded MoS2 and WSe2 nanosheets were investigated in detail. The geometric structures, adsorption energies, projected density of states (PDOS), charge density differences and band structures of the considered heterostructures are discussed. 4
2. Methods and calculation models The calculations presented in this work were performed based on the density functional theory (DFT) [34, 35] employing the electronic structure SIESTA code [36]. To expand the electronic density, the double-zeta polarized (DZP) basis sets were used with norm-conserving TrouillerMartins pseudopotential [37]. GDIS program was used to build the extended supercell from the primitive unit cell and to model the adsorption systems [38]. In order to obtain the Hamiltonian matrix elements, a grid in real space that is achieved using a mesh cutoff of 400 Ry is applied. The convergence criterion for total energy was set to 10-4 eV and the remaining force acting on each atom is less than 0.02 eV/Å. A vacuum space of 20 Å is applied, which is very important to avoid the interaction between the adjacent supercells. For structural relaxations, a k-point mesh of 5 × 5 × 1 was used, while for the electronic structure calculations like the density of states and band structures, the k-point sampling of 10 × 10 × 1 was considered [39]. The generalized gradient approximation (GGA) potential [40, 41], parameterized by the Perdew–Burke–Ernzerhof (PBE) was used to treat the exchange-correlation functional [42]. To visualize the charge density plots and illustration of adsorption configurations, VESTA package was used [43]. The long range van der Waals (vdW) interactions plays a key role in describing the interaction between heterostructures. Thus, the vdW correction was performed using the VDWDRSLL functional to include the effect of vdW in the calculations. We have built a 5×5×1 supercell for pristine MoS2 and WSe2 monolayers and studied the adsorption of C60 fullerene on its surfaces. In the case of Ag-decorated and Au-decorated MoS2 monolayer, the supercell of 4×4×1 was considered in order to place the C60 molecule on the middle of transition metal decorated system. The adsorption energy of C60 molecule on the surface of pristine and transition metal decorated MoS2 monolayer is calculated as the following equation: 5
Ead = E (MoS2 + C60 molecule) - E MoS2 – E C60 molecule Where, E (MoS2
+ C60 molecule)
(1)
denotes the total energy of the MoS2 system with adsorbed C60
molecule, and - E MoS2 refers to that of bare MoS2. E
C60 molecule
also indicates the total energy of
isolated C60 molecule. 3. Results and Discussion 3.1. Structural and electronic properties of pristine and Ag/Au-decorated MoS2 In this work, we first tend to check the structural parameters and electronic properties of pristine and Ag/Au embedded MoS2 monolayers. The optimized geometric structures of the pristine MoS2, Ag/Au embedded MoS2 and WSe2 monolayers in different views were illustrated in Figures 1-2. To explore the electronic properties of transition metal embedded MoS2 monolayers, we have calculated their electronic band structures. The relevant band structure plots were shown in Figure 3. As can be seen from Figure 3a, the pristine MoS2 monolayer is a direct band gap semiconductor with the calculated band gap of about 1.67 eV, which are very close to the other reported computational results [44, 45]. In order to model the Ag/Au decorated MoS2 system, we have embedded one Ag/Au atom into the structure of 4×4×1 supercell of MoS2 monolayer to construct Ag/Au-MoS2 coupled systems. It was found that Ag/Au atom strongly adsorbs on the MoS2 monolayer and forms covalent bonds with the three neighboring sulfur atoms. The adsorptions of Ag and Au atoms on the MoS2 monolayer were calculated to be -2.44 eV and -2.12 eV, respectively. These negative adsorption energies represent that the process is exothermic and energy favorable. Thus, the adsorption of Ag and Au atoms on the MoS2 system leads to the stable configurations with the computed Ag-S and Au-S bond lengths of 2.61 Å and 2.72 Å, respectively.
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Besides this, Ag adsorption on the MoS2 monolayer is more stable than Au adsorption, as confirmed by lower Ag-S distance in MoS2-Ag system than Au-S distance in MoS2-Au system. The band structure plots of the Ag-embedded and Au-embedded MoS2 monolayers were depicted in Figure 3b-c. As can be seen, Ag-embedded MoS2 monolayer represents metallic behavior because electronic bands appeared near the Fermi level. Particularly, one of these bands crossed the Fermi level, which indicated that the Ag-embedded MoS2 system is metallic (See Figure 3b). Similarly, Au-embedded MoS2 system exhibit metallic behavior due to the same reason as in the case of Ag-embedded system. The impurity energy band near the Fermi level can be ascribed to the Ag/Au atoms embedded in the MoS2 monolayer inducing metallicity in the system. As analogous of MoS2, we have also calculated the electronic properties of WSe2 monolayer to evaluate its ability for the adsorption and trapping of C60 fullerenes. The optimized structure of the pristine and Au-embedded WSe2 monolayers and the corresponding band structure plot were shown in Figures 2 and 3, respectively. Similar to MoS2, WSe2 monolayer also represents semiconductor behavior with the direct band gap of about 1.56 eV, in reasonable agreement with the other theoretical works [46]. Interestingly, Au-embedded WSe2 represents semiconductor characteristics and the Fermi level is located between the valence band maximum and the conduction band minimum. 3.2. Adsorption of C60 molecule on the pristine and Ag/Au embedded MoS2 Next, we turn to investigate the adsorption of C60 fullerene on the surface of pristine and Ag/Au embedded MoS2 monolayers. The optimized geometric structures of the pristine MoS2/C60 heterostructures in both top and side views with and without the inclusion of van der Waals (vdW) interaction were shown in Figure 4 along with the total electron density distribution plots. The 7
calculated equilibrium distance between C60 fullerene and the surface top atoms of the MoS2 monolayer is 3.06 Å, which is very close to that between the graphene sheet and other materials (For instance 2.85 Å for TiO2(001)/graphene composite system [47, 48]). The smaller distance shows the strong interaction between MoS2 sheet and C60 fullerene. We found that, after relaxation of whole system, the MoS2 sheet and C60 fullerene are almost unaltered, which indicates the van der Waals (vdW) characteristics of the interaction between pristine MoS2 and C60 fullerene rather than covalent interaction, consistent with the reported results [49]. Geometry optimizations have been carried out for all of the systems according to the conjugate gradient (CG) method. Here, we calculated the adsorption energy to discuss about the stability of the composite system. The calculated adsorption energy for C60 fullerene on pristine MoS2 nanosheet is about 1.75 eV, which is very close to the reported results [50]. This negative adsorption energy suggests that the adsorption of C60 on the pristine MoS2 monolayer is stable. The charge density differences (CDD plots) were also calculated in this work to further comment on the electronic properties of the systems. By evaluating the CDD plots, one can describe the charge redistribution between the interacting atoms in an adsorption reaction. The charge density difference is defined as follows: ∆ρ = ρ (MoS2 + C60 molecule) - ρ (MoS2) – ρ (C60 molecule)
(2)
where, ρ (MoS2 + C60 molecule) and ρ (MoS2), are the total electron densities of MoS2 sheet with adsorbed C60 molecule and the perfect MoS2 sheets, respectively and ρ (gas molecule) refers to the electron density of isolated C60 molecule. The charge density difference plots for the pristine MoS2/C60 heterostructure were displayed in Figure 5. This figure shows that the charges were mainly accumulated on the adsorbed C60 molecule. There is also some electron density at the junction point of MoS2 and C60, indicating the weak vdW interaction between them.
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The optimized geometric structures of the Ag-embedded and Au-embedded MoS2/C60 heterostructures in top and side views were displayed in Figures 6 and 7, respectively. We have also presented the optimized geometry configurations of pristine and Au-decorated WSe2/C60 heterostructures in Figure 8. It can be seen from these figures that Ag and Au atoms show a strong covalent interaction with the carbon atoms of C60 fullerene. The adsorption energy results indicate that C60 interaction with Ag-embedded and Au-embedded MoS2 monolayers is more energetically favorable than that with pristine MoS2 system. Therefore, transition metal (Ag, Au) embedding strengthens the interaction of MoS2 nanosheet with C60 fullerene. The calculated Ag-C and Au-C distances were also found to be 2.85 Å and 2.85 Å, respectively. For C60 adsorption on the surface of WSe2 monolayer, the interaction with Au-decorated monolayer was also found to be more favorable in energy that that with perfect one. The CDD plots and the corresponding total electron densities for Ag-embedded and Au-embedded MoS2/C60 heterostructures were also shown in Figures 9 and 10, respectively. The accumulation of electron density at the interface region indicates the covalent nature of the interaction between Ag/Au and C atoms. This formation of covalent bonds can be also confirmed by the projected density of states (PDOS) plots (Figure 11). The large overlaps between the PDOS plots of the Ag/Au and C atoms represent the formation of chemical bonds between these atoms. In an effort to better comprehend and exploit the electronic properties of the hybrid MoS2/C60 heterostructures, we have presented the band structure plots for the studied systems. The band structure plots for pristine MoS2/C60 systems with and without the inclusion of vdW interaction were shown in Figure 12a-b. Our electronic structure calculations show that the band structures obtained by GGA-PBE and VDW-DRSLL calculations are almost close to each other for the pristine system. For Ag-decorated and Au-decorated MoS2/C60 heterostructures, the relevant band 9
structures were displayed in Figures 13 and 14. In contrast, in these cases, the calculated band structure plots using the GGA-PBE and VDW-DRSLL methods are principally different, which indicates the dominant effect of vdW interaction. Due to the important role of vdW interaction on the electronic properties of vdW heterostructures, we only discuss about the band structures obtained by VDW-DRSLL method. In the case of both Ag-embedded and Au-embedded MoS2/C60 hybrid systems, we can see that the Fermi level is very close to the conduction band minimum (CBM), and one of the electronic bands crossed the Fermi level. This indicates the metallic behavior of the Ag-embedded and Au-embedded MoS2/C60 heterostructures. The band structures of the pristine and Au-decorated WSe2/C60 heterostructures were also illustrated in Figure 15a-b. As can be seen, the pristine WSe2/C60 system exhibits a semiconductor characteristics, while the Au-decorated one shows metallicity due to the insertion of some electronic bands inside the band gap of WSe2/C60 system.
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4. Conclusions In this paper, using the density functional theory calculations, we have investigated the structural and electronic properties of the pristine and Ag/Au embedded MoS2 and WSe2 van der Waals heterostructures. The results suggest that the interaction of C60 fullerene with Ag-embedded and au-embedded MoS2 monolayers is stronger than that with pristine ones, in accordance with the smaller distance of Ag-embedded and Au-embedded MoS2 with C60 than the pristine monolayer. Besides, Au-embedded WSe2 monolayer interacts with C60 fullerene more strongly. The accumulation of electron density at the junction region between Ag/Au embedded MoS2/WSe2 and C60 fullerene indicates the formation of covalent bonds between the C and Ag/Au atoms. This can be evidenced by the overlaps of PDOS spectra of C and Ag/Au atoms. For Ag-embedded and Auembedded MoS2/C60 heterostructure systems, the Fermi level is very close to the conduction band minimum (CBM), and one of the electronic bands crossed the Fermi level. This indicates the metallic behavior of the Ag-embedded and Au-embedded MoS2/C60 heterostructures. Our findings provide a theoretical basis for emerging novel heterostructures based on transition metal dichalcogenides and fullerenes with possible application in nanoscale devices.
Acknowledgements This work was financed by the National Science Foundation of China (Grant No.51979078).
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Table 1. Adsorption energies (Ead in eV) and interlayer distances (Din in Å) for different heterostructure systems of MoS2/C60 and WSe2/C60. Configuration
Ead
Din
MoS2/C60
-1.75
3.06
Ag-MoS2/C60
-2.22
2.36
Au-MoS2/C60
-2.14
2.43
WSe2/C60
-1.86
3.04
Au-WSe2/C60
-2.16
2.42
18
a
b
c
Figure 1. Optimized structures of the (a) pristine MoS2, (b) Ag-embedded MoS2 and (c) Au-embedded MoS2 monolayers in both top and side views. Colors represent atoms accordingly: Mo in green, S in light yellow, Ag in gray and Au in dark yellow.
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b
a
Figure 2. Optimized structures of the (a) pristine WSe2 and (b) Au-embedded monolayers in both top and side views. Colors represent atoms accordingly: W in blue, Se in orange and Au in yellow.
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a
c
b
d
e
Figure 3. Electronic band structure plots of the (a) pristine MoS2, (b) Ag-embedded MoS2, (c) Au-embedded MoS2 and (d) pristine WSe2 and (e) Au-embedded WSe2 monolayers. The Fermi level is set to zero and denoted by a cyan solid line.
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Without vdW
With vdW
Figure 4. Optimized geometry configurations of MoS2/C60 heterostructures with and without the inclusion of van der Waals (vdW) interaction and the corresponding total electron density distribution plots.
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Figure 5. Charge density distribution plots for MoS2/C60 heterostructures. The charge accumulation and depletion regions were represented by a yellow and cyan colors, respectively.
23
Without vdW
With vdW Figure 6. Optimized geometry configurations of Ag-decorated MoS2/C60 heterostructures with and without the inclusion of van der Waals (vdW) interaction and the corresponding total electron density distribution plots.
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Without vdW
With vdW Figure 7. Optimized geometry configurations of Au-decorated MoS2/C60 heterostructures with and without the inclusion of van der Waals (vdW) interaction and the corresponding total electron density distribution plots.
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Pristine WSe2/C60
Au-embedded WSe2/C60 Figure 8. Optimized structures of the pristine and Au-decorated WSe2/C60 heterostructures. Colors represent atoms accordingly: W in blue, Se in yellow, Au in yellow and C in gray.
26
b
a
c Figure 9. Charge density difference (a-b) and total electron density distribution (c) plots for Ag -decorated MoS2/C60 heterostructures.
27
b
a
c Figure 10. Charge density difference (a-b) and total electron density distribution (c) plots for Au -decorated MoS2/C60 heterostructures.
28
Figure 11. Projected density of states for Ag and Au atoms of Ag/Au decorated MoS 2 and carbon atoms of C60 molecule.
29
a
b
Figure 12. Electronic band structure plots of the pristine MoS2/C60 heterostructures with (a) and without (b) the inclusion of van der Waals (vdW) interaction.
30
Figure 13. Electronic band structure plots of the Ag-decorated MoS2/C60 heterostructures with (a) and without (b) the inclusion of van der Waals (vdW) interaction.
31
a
b
Figure 14. Electronic band structure plots of the Au-decorated MoS2/C60 heterostructures with (a) and without (b) the inclusion of van der Waals (vdW) interaction.
32
a
b
Figure 15. Electronic band structure plots of the pristine (a) and Au -decorated (b) WSe2/C60 heterostructures.
33
Graphical abstract
34
Highlights: 1. The electronic properties of MoS2/C60 and WSe2/C60 heterostructures were studied using the DFT calculations. 2. C60 interaction with Ag/Au-embedded monolayers is stronger than that with pristine ones. 3. The CDD plots indicate the accumulation of electron density at the junction point of heterostructures.
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S0169-4332(19)33144-7 https://doi.org/10.1016/j.apsusc.2019.144328 APSUSC 144328
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
31 July 2019 30 August 2019 7 October 2019
Please cite this article as: X. Lu, M. Cui, X. Pan, P. Wang, L. Sun, Investigation of the structural and electronic properties of pristine and Au-embedded MoS2/C60 and WSe2/C60 van der Waals heterostructures: A firstprinciples study, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144328
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© 2019 Published by Elsevier B.V.
Investigation of the structural and electronic properties of pristine and Au-embedded MoS2/C60 and WSe2/C60 van der Waals heterostructures: A first-principles study
Xiaohui Lu a*, Mingxuan Cui a, Xicai Pan b, Peifang Wang a, Lingjie Sun c
a. School of Earth Science and Engineering, Ministry of Education, Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Hohai University, Nanjing, 210098, China b. Institute of Soil Sciences, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing, China c. School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, 410004, China
* Corresponding Author: [email protected] (X. Lu)
1
Abstract The interaction of C60 molecule with pristine and Au-decorated MoS2 and WSe2 nanosheets were investigated using the density functional theory calculations. The results suggest that adsorption of C60 molecule on the surface of Au-embedded MoS2 is more energetically favorable than that on the pristine MoS2 sheet. Thus, Au-embedded MoS2 systems can interact with C60 molecule more strongly. The interaction of C60 molecule with WSe2 nanosheet was also found to be an energy favorable adsorption process. The large overlaps between the projected density of states of the carbon and Ag/Au atoms indicate the covalent nature of the interaction between them, manifested by the newly formed chemical bonds at the interface. This formation of covalent bonds was also revealed by the charge density difference plots of the considered systems. There is some charge accumulation between the newly formed Au-C bonds, indicating the covalent characteristics of the interaction. Based on the band structure plots, we found that the Agembedded and Au-embedded MoS2/C60 heterostructures show metallic behavior, whereas the pristine heterostructure exhibit semiconductor characteristics.
Keywords: Density functional theory; Ag-embedded MoS2/C60; Au-embedded MoS2/C60; WSe2/C60; Band structure
2
1. Introduction New advances in materials science and engineering have been attained by proposing novel 2D materials that are constructed from the various kinds of structures, which leads to some unique electronic, magnetic and optical properties. By minimizing the size of devices, the optical and electronic properties of low-dimensional materials can be effectively modulated. Research in this field has aroused a large number of attention and becomes a hot topic for experimental and theoretical investigations [1-3]. After the successful fabrication of graphene in the laboratory, which has unique physical properties as a 2D material, lots of investigations have been devoted to development of graphene-like 2D layered nanomaterials and heterostructure systems composed of these 2D monolayers. In these heterostructures, there are strong covalent bonds between the atoms of each monolayer, while the interlayer interaction between these monolayers is described based on weak van der Waals (vdW) forces [4-6]. Furthermore, these 2D heterostructures may display unique electronic properties compared with the bulk counterparts such as semiconductor/metal phase transitions, direct/indirect band gap transitions, and so on. Because of these outstanding properties, 2D layered materials are of great importance to construct vdW heterojunctions with extensive applications in the next generation nanoelectronic and optoelectronic devices [7-9]. In the past few years, carbonaceous materials such as graphene, fullerene, and carbon nanotubes have attracted significant interests owing to their special structures and extraordinary electronic properties [10, 11], which lead to the great improvements on the photocatalytic performances of semiconductors. For example, the combination of C3N4 with carbonaceous materials gives rise to a novel heterojunction photocatalysts with unique properties. For instance, heterostructures composed of graphene and C3N4, fabricated by the impregnation–chemical reduction strategy, exhibit great visible light photocatalytic activity for some processes such as hydrogen production 3
[12]. Recent investigations have revealed that composite systems containing graphene and many two-dimensional materials greatly enhance the electronic and optoelectronic properties of 2D materials [13-15]. Hence, we aim at studying the electronic and structural properties of molybdenum disulfide (MoS2) embedded with C60 molecules because of its unique band structures and consequently satisfied band gap. To the best of our knowledge, no theoretical study has been performed on the exploration of the electronic, magnetic and structural properties of MoS2/C60 hybrid nanosystems. However, there exists some investigations on the interaction of different 2D nanomaterials with external agents (gas molecules/transitional metals) focusing on their applicability in gas sensor devices [16-20]. MoS2/graphene heterostructures and graphene-like MoS2/amorphous carbon hybrid systems have been fabricated in the laboratory by Chang et al. [21-22]. The first principles calculations indicate that C60-interfaced TiO2 in both the covalent linking and mechanical mixture cannot form a promising photovoltaic heterojunction [23], while B- or N doped C60/MoS2 or C60/WS2 heterostructures can form an effective photovoltaic heterojunction [24]. The novel electronic and structural properties of different heterostructures consisting of transition metal dichalcogenides and other 2D materials have been systematically explored [25-30]. For instance, Abbasi et al. [3133] have examined the gas sensing properties of heterostructures composed of TiO2 and WSe2/Stanene monolayers. In this work, we carried out a density functional theory (DFT) study to explore the structural and electronic properties of a van der Waals heterostructure constructed from C60 fullerene and MoS2 or WSe2 monolayers. The interaction of C60 fullerene with both pristine and Au-embedded MoS2 and WSe2 nanosheets were investigated in detail. The geometric structures, adsorption energies, projected density of states (PDOS), charge density differences and band structures of the considered heterostructures are discussed. 4
2. Methods and calculation models The calculations presented in this work were performed based on the density functional theory (DFT) [34, 35] employing the electronic structure SIESTA code [36]. To expand the electronic density, the double-zeta polarized (DZP) basis sets were used with norm-conserving TrouillerMartins pseudopotential [37]. GDIS program was used to build the extended supercell from the primitive unit cell and to model the adsorption systems [38]. In order to obtain the Hamiltonian matrix elements, a grid in real space that is achieved using a mesh cutoff of 400 Ry is applied. The convergence criterion for total energy was set to 10-4 eV and the remaining force acting on each atom is less than 0.02 eV/Å. A vacuum space of 20 Å is applied, which is very important to avoid the interaction between the adjacent supercells. For structural relaxations, a k-point mesh of 5 × 5 × 1 was used, while for the electronic structure calculations like the density of states and band structures, the k-point sampling of 10 × 10 × 1 was considered [39]. The generalized gradient approximation (GGA) potential [40, 41], parameterized by the Perdew–Burke–Ernzerhof (PBE) was used to treat the exchange-correlation functional [42]. To visualize the charge density plots and illustration of adsorption configurations, VESTA package was used [43]. The long range van der Waals (vdW) interactions plays a key role in describing the interaction between heterostructures. Thus, the vdW correction was performed using the VDWDRSLL functional to include the effect of vdW in the calculations. We have built a 5×5×1 supercell for pristine MoS2 and WSe2 monolayers and studied the adsorption of C60 fullerene on its surfaces. In the case of Ag-decorated and Au-decorated MoS2 monolayer, the supercell of 4×4×1 was considered in order to place the C60 molecule on the middle of transition metal decorated system. The adsorption energy of C60 molecule on the surface of pristine and transition metal decorated MoS2 monolayer is calculated as the following equation: 5
Ead = E (MoS2 + C60 molecule) - E MoS2 – E C60 molecule Where, E (MoS2
+ C60 molecule)
(1)
denotes the total energy of the MoS2 system with adsorbed C60
molecule, and - E MoS2 refers to that of bare MoS2. E
C60 molecule
also indicates the total energy of
isolated C60 molecule. 3. Results and Discussion 3.1. Structural and electronic properties of pristine and Ag/Au-decorated MoS2 In this work, we first tend to check the structural parameters and electronic properties of pristine and Ag/Au embedded MoS2 monolayers. The optimized geometric structures of the pristine MoS2, Ag/Au embedded MoS2 and WSe2 monolayers in different views were illustrated in Figures 1-2. To explore the electronic properties of transition metal embedded MoS2 monolayers, we have calculated their electronic band structures. The relevant band structure plots were shown in Figure 3. As can be seen from Figure 3a, the pristine MoS2 monolayer is a direct band gap semiconductor with the calculated band gap of about 1.67 eV, which are very close to the other reported computational results [44, 45]. In order to model the Ag/Au decorated MoS2 system, we have embedded one Ag/Au atom into the structure of 4×4×1 supercell of MoS2 monolayer to construct Ag/Au-MoS2 coupled systems. It was found that Ag/Au atom strongly adsorbs on the MoS2 monolayer and forms covalent bonds with the three neighboring sulfur atoms. The adsorptions of Ag and Au atoms on the MoS2 monolayer were calculated to be -2.44 eV and -2.12 eV, respectively. These negative adsorption energies represent that the process is exothermic and energy favorable. Thus, the adsorption of Ag and Au atoms on the MoS2 system leads to the stable configurations with the computed Ag-S and Au-S bond lengths of 2.61 Å and 2.72 Å, respectively.
6
Besides this, Ag adsorption on the MoS2 monolayer is more stable than Au adsorption, as confirmed by lower Ag-S distance in MoS2-Ag system than Au-S distance in MoS2-Au system. The band structure plots of the Ag-embedded and Au-embedded MoS2 monolayers were depicted in Figure 3b-c. As can be seen, Ag-embedded MoS2 monolayer represents metallic behavior because electronic bands appeared near the Fermi level. Particularly, one of these bands crossed the Fermi level, which indicated that the Ag-embedded MoS2 system is metallic (See Figure 3b). Similarly, Au-embedded MoS2 system exhibit metallic behavior due to the same reason as in the case of Ag-embedded system. The impurity energy band near the Fermi level can be ascribed to the Ag/Au atoms embedded in the MoS2 monolayer inducing metallicity in the system. As analogous of MoS2, we have also calculated the electronic properties of WSe2 monolayer to evaluate its ability for the adsorption and trapping of C60 fullerenes. The optimized structure of the pristine and Au-embedded WSe2 monolayers and the corresponding band structure plot were shown in Figures 2 and 3, respectively. Similar to MoS2, WSe2 monolayer also represents semiconductor behavior with the direct band gap of about 1.56 eV, in reasonable agreement with the other theoretical works [46]. Interestingly, Au-embedded WSe2 represents semiconductor characteristics and the Fermi level is located between the valence band maximum and the conduction band minimum. 3.2. Adsorption of C60 molecule on the pristine and Ag/Au embedded MoS2 Next, we turn to investigate the adsorption of C60 fullerene on the surface of pristine and Ag/Au embedded MoS2 monolayers. The optimized geometric structures of the pristine MoS2/C60 heterostructures in both top and side views with and without the inclusion of van der Waals (vdW) interaction were shown in Figure 4 along with the total electron density distribution plots. The 7
calculated equilibrium distance between C60 fullerene and the surface top atoms of the MoS2 monolayer is 3.06 Å, which is very close to that between the graphene sheet and other materials (For instance 2.85 Å for TiO2(001)/graphene composite system [47, 48]). The smaller distance shows the strong interaction between MoS2 sheet and C60 fullerene. We found that, after relaxation of whole system, the MoS2 sheet and C60 fullerene are almost unaltered, which indicates the van der Waals (vdW) characteristics of the interaction between pristine MoS2 and C60 fullerene rather than covalent interaction, consistent with the reported results [49]. Geometry optimizations have been carried out for all of the systems according to the conjugate gradient (CG) method. Here, we calculated the adsorption energy to discuss about the stability of the composite system. The calculated adsorption energy for C60 fullerene on pristine MoS2 nanosheet is about 1.75 eV, which is very close to the reported results [50]. This negative adsorption energy suggests that the adsorption of C60 on the pristine MoS2 monolayer is stable. The charge density differences (CDD plots) were also calculated in this work to further comment on the electronic properties of the systems. By evaluating the CDD plots, one can describe the charge redistribution between the interacting atoms in an adsorption reaction. The charge density difference is defined as follows: ∆ρ = ρ (MoS2 + C60 molecule) - ρ (MoS2) – ρ (C60 molecule)
(2)
where, ρ (MoS2 + C60 molecule) and ρ (MoS2), are the total electron densities of MoS2 sheet with adsorbed C60 molecule and the perfect MoS2 sheets, respectively and ρ (gas molecule) refers to the electron density of isolated C60 molecule. The charge density difference plots for the pristine MoS2/C60 heterostructure were displayed in Figure 5. This figure shows that the charges were mainly accumulated on the adsorbed C60 molecule. There is also some electron density at the junction point of MoS2 and C60, indicating the weak vdW interaction between them.
8
The optimized geometric structures of the Ag-embedded and Au-embedded MoS2/C60 heterostructures in top and side views were displayed in Figures 6 and 7, respectively. We have also presented the optimized geometry configurations of pristine and Au-decorated WSe2/C60 heterostructures in Figure 8. It can be seen from these figures that Ag and Au atoms show a strong covalent interaction with the carbon atoms of C60 fullerene. The adsorption energy results indicate that C60 interaction with Ag-embedded and Au-embedded MoS2 monolayers is more energetically favorable than that with pristine MoS2 system. Therefore, transition metal (Ag, Au) embedding strengthens the interaction of MoS2 nanosheet with C60 fullerene. The calculated Ag-C and Au-C distances were also found to be 2.85 Å and 2.85 Å, respectively. For C60 adsorption on the surface of WSe2 monolayer, the interaction with Au-decorated monolayer was also found to be more favorable in energy that that with perfect one. The CDD plots and the corresponding total electron densities for Ag-embedded and Au-embedded MoS2/C60 heterostructures were also shown in Figures 9 and 10, respectively. The accumulation of electron density at the interface region indicates the covalent nature of the interaction between Ag/Au and C atoms. This formation of covalent bonds can be also confirmed by the projected density of states (PDOS) plots (Figure 11). The large overlaps between the PDOS plots of the Ag/Au and C atoms represent the formation of chemical bonds between these atoms. In an effort to better comprehend and exploit the electronic properties of the hybrid MoS2/C60 heterostructures, we have presented the band structure plots for the studied systems. The band structure plots for pristine MoS2/C60 systems with and without the inclusion of vdW interaction were shown in Figure 12a-b. Our electronic structure calculations show that the band structures obtained by GGA-PBE and VDW-DRSLL calculations are almost close to each other for the pristine system. For Ag-decorated and Au-decorated MoS2/C60 heterostructures, the relevant band 9
structures were displayed in Figures 13 and 14. In contrast, in these cases, the calculated band structure plots using the GGA-PBE and VDW-DRSLL methods are principally different, which indicates the dominant effect of vdW interaction. Due to the important role of vdW interaction on the electronic properties of vdW heterostructures, we only discuss about the band structures obtained by VDW-DRSLL method. In the case of both Ag-embedded and Au-embedded MoS2/C60 hybrid systems, we can see that the Fermi level is very close to the conduction band minimum (CBM), and one of the electronic bands crossed the Fermi level. This indicates the metallic behavior of the Ag-embedded and Au-embedded MoS2/C60 heterostructures. The band structures of the pristine and Au-decorated WSe2/C60 heterostructures were also illustrated in Figure 15a-b. As can be seen, the pristine WSe2/C60 system exhibits a semiconductor characteristics, while the Au-decorated one shows metallicity due to the insertion of some electronic bands inside the band gap of WSe2/C60 system.
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4. Conclusions In this paper, using the density functional theory calculations, we have investigated the structural and electronic properties of the pristine and Ag/Au embedded MoS2 and WSe2 van der Waals heterostructures. The results suggest that the interaction of C60 fullerene with Ag-embedded and au-embedded MoS2 monolayers is stronger than that with pristine ones, in accordance with the smaller distance of Ag-embedded and Au-embedded MoS2 with C60 than the pristine monolayer. Besides, Au-embedded WSe2 monolayer interacts with C60 fullerene more strongly. The accumulation of electron density at the junction region between Ag/Au embedded MoS2/WSe2 and C60 fullerene indicates the formation of covalent bonds between the C and Ag/Au atoms. This can be evidenced by the overlaps of PDOS spectra of C and Ag/Au atoms. For Ag-embedded and Auembedded MoS2/C60 heterostructure systems, the Fermi level is very close to the conduction band minimum (CBM), and one of the electronic bands crossed the Fermi level. This indicates the metallic behavior of the Ag-embedded and Au-embedded MoS2/C60 heterostructures. Our findings provide a theoretical basis for emerging novel heterostructures based on transition metal dichalcogenides and fullerenes with possible application in nanoscale devices.
Acknowledgements This work was financed by the National Science Foundation of China (Grant No.51979078).
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Table 1. Adsorption energies (Ead in eV) and interlayer distances (Din in Å) for different heterostructure systems of MoS2/C60 and WSe2/C60. Configuration
Ead
Din
MoS2/C60
-1.75
3.06
Ag-MoS2/C60
-2.22
2.36
Au-MoS2/C60
-2.14
2.43
WSe2/C60
-1.86
3.04
Au-WSe2/C60
-2.16
2.42
18
a
b
c
Figure 1. Optimized structures of the (a) pristine MoS2, (b) Ag-embedded MoS2 and (c) Au-embedded MoS2 monolayers in both top and side views. Colors represent atoms accordingly: Mo in green, S in light yellow, Ag in gray and Au in dark yellow.
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b
a
Figure 2. Optimized structures of the (a) pristine WSe2 and (b) Au-embedded monolayers in both top and side views. Colors represent atoms accordingly: W in blue, Se in orange and Au in yellow.
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a
c
b
d
e
Figure 3. Electronic band structure plots of the (a) pristine MoS2, (b) Ag-embedded MoS2, (c) Au-embedded MoS2 and (d) pristine WSe2 and (e) Au-embedded WSe2 monolayers. The Fermi level is set to zero and denoted by a cyan solid line.
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Without vdW
With vdW
Figure 4. Optimized geometry configurations of MoS2/C60 heterostructures with and without the inclusion of van der Waals (vdW) interaction and the corresponding total electron density distribution plots.
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Figure 5. Charge density distribution plots for MoS2/C60 heterostructures. The charge accumulation and depletion regions were represented by a yellow and cyan colors, respectively.
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Without vdW
With vdW Figure 6. Optimized geometry configurations of Ag-decorated MoS2/C60 heterostructures with and without the inclusion of van der Waals (vdW) interaction and the corresponding total electron density distribution plots.
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Without vdW
With vdW Figure 7. Optimized geometry configurations of Au-decorated MoS2/C60 heterostructures with and without the inclusion of van der Waals (vdW) interaction and the corresponding total electron density distribution plots.
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Pristine WSe2/C60
Au-embedded WSe2/C60 Figure 8. Optimized structures of the pristine and Au-decorated WSe2/C60 heterostructures. Colors represent atoms accordingly: W in blue, Se in yellow, Au in yellow and C in gray.
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b
a
c Figure 9. Charge density difference (a-b) and total electron density distribution (c) plots for Ag -decorated MoS2/C60 heterostructures.
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b
a
c Figure 10. Charge density difference (a-b) and total electron density distribution (c) plots for Au -decorated MoS2/C60 heterostructures.
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Figure 11. Projected density of states for Ag and Au atoms of Ag/Au decorated MoS 2 and carbon atoms of C60 molecule.
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a
b
Figure 12. Electronic band structure plots of the pristine MoS2/C60 heterostructures with (a) and without (b) the inclusion of van der Waals (vdW) interaction.
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Figure 13. Electronic band structure plots of the Ag-decorated MoS2/C60 heterostructures with (a) and without (b) the inclusion of van der Waals (vdW) interaction.
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a
b
Figure 14. Electronic band structure plots of the Au-decorated MoS2/C60 heterostructures with (a) and without (b) the inclusion of van der Waals (vdW) interaction.
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a
b
Figure 15. Electronic band structure plots of the pristine (a) and Au -decorated (b) WSe2/C60 heterostructures.
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Graphical abstract
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Highlights: 1. The electronic properties of MoS2/C60 and WSe2/C60 heterostructures were studied using the DFT calculations. 2. C60 interaction with Ag/Au-embedded monolayers is stronger than that with pristine ones. 3. The CDD plots indicate the accumulation of electron density at the junction point of heterostructures.
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