Electronic, optical and mechanical properties of graphene/MoS₂ nanocomposite

 Journal of Science and Technique - N.209 (7-2020) - Le Quy Don Technical University 
 ELECTRONIC, OPTICAL AND MECHANICAL PROPERTIES 
 OF GRAPHENE/MoS2 NANOCOMPOSITE 
 Nguyen Van Chuong1,*, Nguyen Dinh Chien1, Le Minh Duc1, 
 Nguyen Ngoc Hieu2, Nguyen Son Tung3 
 1Le Quy Don Technical University; 
 2Duy Tan University; 
 3Hanoi University of Industry 
 Abstract 
 In this work, we construct an ultrathin graphene/MoS2 nanocomposite and investigate 
 systematically its electronic, optical and mechanical properties using first-principles 
 calculations based on density functional theory. Our results show that graphene and MoS2 
 layers in their corresponding graphene/MoS2 nanocomposite are bonded mainly via the 
 weak van der Waals forces, which are not enough to modify the intrinsic properties of the 
 constituent monolayers, thus the electronic properties are well preserved. Moreover, the 
 optical and mechanical properties of the graphene/MoS2 nanocomposite are enhanced as 
 compared with those of individual constituent graphene and MoS2 monolayers. The 
 maximum of absorption intensity can reach up to 2.5×105 cm-1. Moreover, the Young’s 
 modulus of nanocomposite increases up to 487.2 N/m2. These findings demonstrate that the 
 formation of the graphene/MoS2 nanocomposite could effectively be used to enhance the 
 electronic, optical and mechanical performances of both graphene and MoS2 monolayers. 
 Keywords: Graphene/MoS2 nanocomposite; two-dimensional materials; DFT calculations. 
1. Introduction 
 Since the discovery in 2004 by Geim and co-workers, graphene [1] has become one 
of the materials that have attracted both theoretical and experimental scientists due to its 
extraordinary physical properties. However, the application of graphene to technology, 
especially in the field of electronic and optoelectronic devices, still faces certain 
difficulties, in which the cause may be due to graphene having zero energy gap [2] and 
incompatibility between graphene and silicon electronic components. So far, there are 
many different ways to change the electronic states of graphene, i.e., to open the energy 
gap near the Fermi level in graphene: (i) the size effect leads to the opening of the energy 
gap in the nanoribbons; (ii) lateral effects and defects; (iii) doping and functionalism 
effects: spurious and functional atoms can change the material properties; (iv) layer 
(thickness) effect: the electronic structure depends strongly on the number of layers. 
* Email: chuong.vnguyen@lqdtu.edu.vn 
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Journal of Science and Technique - N.209 (7-2020) - Le Quy Don Technical University 
 In parallel with finding a way to overcome this limitation of graphene, a new 
research direction has emerged strongly in the last five years. That is looking for 
alternative materials. This new research has focused on 2D materials such as 
phosphorene, antimonene, transition metal dichalcogenides (TMDs), and 
monochalcogenides, etc. Unlike graphene, these 2D materials are semiconductors with 
interesting properties and they become a potential candidate for applications in 
nanotechnology, such as photodetectors [3, 4], field effect transistors (FETs) [5], 
These application potentials have prompted scientists to continue to study the 
outstanding electronic and transport properties of these materials and to explore their 
application potential for designing high-performance optoelectronic nanodevices. 
 An another method currently being investigated is the creation of vdW layered 
nanocomposites from 2D materials, thereby allowing for a better control of the 
electronic and mechanical properties of the constituent monolayers. Nanocomposites of 
2D materials are stacked to create large electric fields originating from the difference in 
work function. Previously, Qiu and co-workers have investigated the optical properties 
of graphene/MoS2 heterostructure by using the density functional theory [6]. Also, the 
mechanical properties of graphene/MoS2 heterostructure have been studied by 
molecular dynamics simulations [7]. In addition, experimental and theoretical studies 
have shown that the extraordinary electronic properties of the constituent materials are 
preserved due to the weak vdW interaction between layers in the nanocomposites. First 
of all, we can mention the successful hybridization between graphene and a variety of 
other 2D semiconductor materials such as graphene/MoS2 [8, 9], graphene/phosphorene 
[10], graphene/GaSe [11], etc. using different methods both experimentally and 
theoretically. Besides, hybridization between two-dimensional materials such as 
arsenene/C3N [12], GaS/MoS2 [13] is increasingly being considered. It can be seen that 
in these vDW nanocomposites, researchers have discovered some interesting properties 
that do not exist in individual constituent monolayers. For vdW nanocomposites, the 
vdW interactions between monolayers can ke ...  of the isolated graphene, MoS2 monolayers and 
their graphene/MoS2 nanocomposite are depicted in Fig. 2. One can observe from 
Fig. 2a that the graphene has a linear relation at the Dirac K point, resulting in the gap-
less semiconductor. On the contrary, MoS2 monolayer displays a direct band gap 
semiconductor, forming between the valence band maximum (VBM) and conduction 
band minimum (CBM) at the K Dirac point, as illustrated in Fig. 2b. When the 
graphene/MoS2 nanocomposite is formed, one can clearly observe that its electronic 
band structure seems to be a combination of that of the individual constituent graphene 
and MoS2 monolayers. It indicates that the electronic properties of both graphene and 
MoS2 monolayers are well preserved in their combined graphene/MoS2 nanocomposite. 
The Dirac cone at the K point of graphene is preserved in such nanocomposite, 
suggesting that its intrinsic electronic characteristics are maintained. More interestingly, 
we find that when the graphene/MoS2 nanocomposite is formed, a tiny band gap of 
10 meV has opened at the Dirac point of graphene, making it suitable for designing 
next-generation high speed optoelectronic nanodevices, such as field-effect transistor, as 
illustrated in Fig. 3. The mechanism of such band gap, opening in graphene is due to the 
symmetry breaking of the sublattice’s graphene. This behavior was also confirmed by 
the experimental report [8, 27]. 
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 Journal of Science and Technique - N.209 (7-2020) - Le Quy Don Technical University 
 Fig. 3. Schematic model of field-effect transistor based on the graphene/MoS2 nanocomposite. 
 More interestingly, when the graphene/MoS2 nanocomposite is formed, it creates 
the metal/semiconductor contact, resulting in the formation of the Schottky or Ohmic 
contact. It should be noted that the performance of nanodevices depends on the 
formation of the Schottky or Ohmic contact in the metal/semiconductor contact. 
Depending of the position of the Fermi energy level, as depicted in Fig. 2c, we can find 
that the graphene/MoS2 nanocomposite forms the Schottky contact. According to the 
Schottky-Mott rule [28], the Schottky barrier height of the n-type and p-type Schottky 
contact can be obtained as: ΦB,n = EF – EVBM, and ΦB,p = ECBM – EF, where EVBM, ECBM 
and EF, respectively, are the positions of the VBM, the CBM and the Fermi level of the 
graphene/MoS2 nanocomposite. Our calculated ΦB,n the graphene/MoS2 nanocomposite 
is 0.49 eV, which is slightly smaller than the ΦB,p of 1.24 eV, indicating that the 
nanocomposite forms the n-type Schottky contact at the ground state. It should be noted 
that the Schottky contact in the graphene/MoS2 nanocomposite is very different from 
traditional metal-semiconductor Schottky one. One is that graphene is adsorbed 
physically on MoS2 monolayer without dangling bonds. In addition, the n-type Schottky 
barrier height of the graphene/MoS2 nanocomposite is still smaller than 
that in other graphene-based nanocomposites, such as graphene/GaN [29], 
graphene/phosphorene [30]. It indicates that the Schottky devices based on the 
graphene/MoS2 nanocomposite will predict to present a better performance than those 
based on the graphene/GaN and graphene/phosphorene. 
 Furthermore, the optical absorption of the nanocomposite is so crucial for the 
efficient utilization of the solar energy efficiency. We hence calculated the optical 
absorption spectra as a function of the photon energy. The optical absorption coefficient 
  is calculated as follows: 
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Journal of Science and Technique - N.209 (7-2020) - Le Quy Don Technical University 
 1
 (  ) 2  2 (  )  2 (  )  (  ) 2 (4) 
 1 2 1 
Here, 1()  and 2 ()  are the real and imaginary part of dielectric functions of materials. 
 Fig. 4. Optical absorption of the graphene/MoS2 nanocomposite. 
 The optical absorption coefficient of the graphene/MoS2 nanocomposite is 
displayed in Fig. 4 along with that of the individual constituent graphene and MoS2 
monolayers. We can find that the graphene/MoS2 nanocomposite exhibits a large 
absorption coefficient than the graphene and MoS2 monolayers. The maximum of 
absorption intensity can reach up to 2.5×105 cm-1. In addition, one can observe that the 
optical band gap of the graphene/MoS2 nanocomposite is still smaller than that of the 
individual constituent graphene and MoS2 monolayers. It is well-known that the optical 
absorption coefficient of MoS2 is quite small, this leads to difficulties in the application 
of MoS2 in optoelectronic devices. The formation of two-layer heterostructures based on 
MoS2 to achieve a high absorption coefficient, as in the case of graphene/MoS2, has 
brought new prospects for the application of MoS2 in the optoelectronics. 
 We now turn to consider the mechanical properties of the graphene/MoS2 
nanocomposite. We first calculate the elastic stiffness constants Cij by using the stress-
strain relationship and the elastic moduli. As above-mentioned, the graphene/MoS2 
nanocomposite has the hexagonal structure, thus we further consider only the values of 
C11 = C22, C12, and C66 in the graphene/MoS2 nanocomposite. The layer modulus of 2D 
system, including graphene/MoS2 nanocomposite can be calculated as follows: 
 1
  2D CC  
 2 11 12
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 Journal of Science and Technique - N.209 (7-2020) - Le Quy Don Technical University 
From this point, the average Young’s modulus (E), Poisson’s ratio ( ) and shear 
modulus (G) can be calculated as follows: 
 2 2
 CCC11 12 12
 EGC ;; 66 
 CC11 11
 Tab. 2. Calculated elastic stiffness constants (N/m), Young’s modulus (N/m), 
 and Poisson’s ratio of the graphene/MoS2 nanocomposite along 
 with those of isolated graphene and MoS2 monolayers. 
 Layer Young’s Poisson’s 
 2D systems C C C 
 11 12 66 modulus modulus ratio 
 Graphene 356.3 62.3 150.5 209.3 345.4 0.17 
 MoS2 131.2 39.2 46.3 85.2 119.5 0.34 
 Graphene/MoS 
 2 512.3 113.4 200.3 312.85 487.2 0.22 
 nanocomposite 
 One can observe from Tab. 2 that the elastic properties of the graphene/MoS2 
nanocomposite are enhanced in comparison with those of the constituent isolated 
graphene and MoS2 monolayers. More interestingly, we can find that the elastic 
properties of such nanocomposite seem to be a combination of those of the constituent 
monolayers. Therefore, we can conclude that when the graphene stacked on the MoS2 to 
form the graphene/MoS2 nanocomposite, its elastic properties, including layer and 
Young’s modulus are better than that of each individual monolayer, making it 
promising candidate for multifunctional nanodevices. 
4. Conclusions 
 In conclusion, we have constructed an ultrathin graphene/MoS2 nanocomposite and 
investigated its electronic, optical and mechanical properties using first principles 
calculations. We find that in the graphene/MoS2 nanocomposite, the intrinsic properties 
of both graphene and MoS2 layers are well preserved because of the weak vdW 
interactions, dominating between graphene and MoS2 monolayers. The graphene/MoS2 
nanocomposite exhibits the enhanced electronic, optical and mechanical properties as 
compared with those of individual constituent graphene and MoS2 monolayers. 
These findings provide an opportunity for the graphene/MoS2 nanocomposite in the 
next-generation nanoelectronic and optoelectronic devices, which can be used to replace 
principal silicon-based devices. 
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Journal of Science and Technique - N.209 (7-2020) - Le Quy Don Technical University 
Acknowledgements 
 This research is funded by the Vietnam National Foundation for Science and 
Technology Development (NAFOSTED) under grant number 103.01-2019.05. 
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Journal of Science and Technique - N.209 (7-2020) - Le Quy Don Technical University 
 NGHIÊN CỨU CÁC TÍNH CHẤT ĐIỆN TỬ, QUANG HỌC 
VÀ CƠ TÍNH CỦA VẬT LIỆU NANO COMPOSITE GRAPHENE/MoS2 
 Tóm tắt: Trong bài báo này, chúng tôi mô phỏng và nghiên cứu các tính chất điện tử, 
quang học và cơ tính của hệ vật liệu màng mỏng nano graphene/MoS2 sử dụng lý thuyết phiếm 
hàm mật độ. Kết quả nghiên cứu cho thấy lực tương tác yếu van der Waals giữa các lớp vật liệu 
giữ cho hệ vật liệu nanocomposite graphene/MoS2 bền vững và không gây phá hủy các tính chất 
điện tử nổi trội của graphene và MoS2 đơn lớp. Bên cạnh đó, chúng tôi thấy rằng các tính chất 
quang học và cơ tính của graphene và MoS2 được tăng cường trong hệ vật liệu nanocomposite. 
Hệ số quang hấp thụ tối đa của hệ có thể đạt 2,5×105 cm-1. Trong khi đó, mô đun đàn hồi Young 
của hệ nanocomposite này tăng lên tới 487,2 N/m2. Các kết quả nghiên cứu này chỉ ra rằng sự 
hình thành hệ vật liệu màng mỏng nanocomposite của graphene và MoS2 là phương pháp hiệu 
dụng để tăng cường các tính chất điện tử, quang học cũng như cơ tính của các vật liệu tiềm 
năng graphene và MoS2. 
 Từ khóa: Vật liệu nanocomposite graphene/MoS2; các vật liệu hai chiều; phương pháp 
phiếm hàm mật độ. 
 Received: 10/02/2020; Revised: 26/7/2020; Accepted for publication: 28/7/2020 
  
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