Document Type: Research Paper
Author
University of Mazandaran
Abstract
Keywords

Mechanics of Advanced Composite Structures 3 (2016) 0913 

Semnan University 
Mechanics of Advanced Composite Structures journal homepage: http://macs.journals.semnan.ac.ir 
Effect of Curvature on the Mechanical Properties of Graphene: A Density Functional Tightbinding Approach
M. Ghorbanzadeh Ahangari
Department of Mechanical engineering, Faculty of Engineering and Technology, University of Mazandaran, Babolsar, Iran
Paper INFO 

ABSTRACT 
Paper history: Received 20151121 Revision Received 20160229 Accepted 20160319 
Due to the high cost of experimental analyses, researchers used atomistic modeling methods for predicting the mechanical behavior of the materials in the fields of nanotechnology. In the present study the SelfConsistent Charge Density Functional TightBinding (SCCDFTB) was used to calculate Young's moduli and average potential energy of the straight and curved graphenes with different curvature widths under axial strain. Also, this method was used to determine the magnitude of the curvature on the aforementioned mechanical properties. From the results it can be concluded that Young's moduli of straight graphene is equal to 1.3 TPa and this mechanical property decreases slowly by decreasing the curvature width of graphenes. Also, the average potential energy and Young's modulus of graphenes decrease with increasing the number of curvature. In next section the Young's moduli of oneatom vacancy and twoatom vacancy defect were calculated and it was found that this mechanical property decreased with increasing the number of atom vacancy in the curved graphene. 



Keywords: Curved graphene Density functional tightbinding Defect Young's modulus



© 2016 Published by Semnan University Press. All rights reserved. 
Graphene is one of the exquisite materials that have stimulated researchers due to its particular properties and applications so as to carry out novel investigations [1]. It is a generally acknowledged fact that three sorts of graphene can be characterized: monolayer graphene, doublelayer graphene and fewlayer graphene containing several layers lower than ten [2]. The monolayer graphene as a twodimensional matter is constituted of carbon which is bonded together with sp2bonded atoms in the form of hexagonal structure [3, 4]. Beyond that, there are multilayer graphene sheets structured with multiple monolayer ones [2]. Compared with Carbon NanoTubes (CNTs) and fullerenes, scientists have turned their eyes onto graphene sheets as a result of their exceptional mechanical and thermal attributes and outstanding electrical conductivity [5, 6]. The significant elite properties of graphene are: ballistic electron carrier [7], bioadaptability and thermal stability [1], quantum Hall effect [8, 9], extremely great Young’s modulus [10], high elasticity [11], superior fracture strength [12], thermal conductivity [13], and particular surface area [14]. Because of these properties, graphene is favorable for a large number of applications in nanoelectronic [7] such as supercapacitors [15], batteries [16], sensors [17] as well as the fact that the fabrication of electronic device can be eased using some kind of graphene sheets prepared from graphite [18]. Furthermore, slim graphene films can be used as electrodes in the solar cells [19]. Coupled with this, due to scant density and great aspect ratio, graphene is one of the eligible entrants for enlarging graphenereinforced composite [4], e.g. it can be scattered throughout the inorganic [20] or polymeric matrix [21] and ceramics [20] to make an ideal composite.
Needless to say, trimming and evolving numerical instruments so as to simulate the mechanical traits could lower the expenses of projects [3]. Despite the fact that multitude theoretical computational investigations have been carried out by researchers to identify the mechanical traits of graphene sheets, the results of them are various due to different theoretical methods they are based upon [2225].
Totally, the mechanical traits of graphene are described utilizing two principal simulation procedures which can be divided into the following classifications: analytical continuum mechanics and atomistic modeling methods. Regarding this fact, it should be added that the analytical continuum modeling is formed utilizing the expanded theories in elasticity [26, 27].
However, in the atomistic modeling methods, the nanomaterial is modeled in nanoscale including various details such as the position of atoms, the interactive forces and the boundary conditions. In contrast, the atomistic modeling is challenged and led to some disadvantages in some circumstances, for instance, it is not performed logically in the models involving a large quantity of atoms, complex formulation, and etc. [28]. It is necessary to point out that the atomistic modeling can be categorized into three principal approaches: (a) extremely classical actuarial mechanics methods like Molecular Dynamic (MD), (b) Monte Carlo (MC), that both of them use the second Newton’s law for nanoscale construction with several thousands atoms [23], and (c) quantum mechanical methods as an abinitio investigation methods for nanoscale system, contain several hundreds atoms. Beyond that, some wellsuited and mixed methods as abinitio techniques exist such as Local Density (LD), Tight Binding Molecular Dynamics (TBMD) and Density Functional Theory (DFT) [29].
The DFT is a circa fast and lowpriced abinitio investigation method which can be utilized to calculate meticulous electronic and constructional attributes of the molecular systems. In comparison to other abinitio approaches, the DFT is a far more capable method for modeling the large systems [30]. In addition, the DFT has more accurate and reliable outputs than those of the MD methods [28].
In order to calculate the mechanical attributes and entire energies of nanostructures, latterly, a density functional theory based tightbinding (DFTB) calculation is used as a practicable method which is parameterized directly utilizing the DFT method [31]. It should be added that the DFTB calculation can be utilized in calculations of large systems, besides; it is worth noting that the DFTB calculation is majorly based upon the firstprinciples rather than other tightbinding models [32]. Although some studies are focused on predicting the mechanical properties of graphene using the DFTB approach, to the best of our knowledge, there is no study on the mechanical properties of the curved graphenes in the previous literatures.
The present study examines the effect of curvature of graphenes on the Young's modulus by SCCDFTB calculations. Furthermore, we consider two other effects on the Young's modulus of the curved graphenes: the amount of curvature and the type of defect in the curved graphenes.
In this study, the geometry optimization and elastic properties were calculated by the SelfConsistent Charge Density Functional TightBinding (SCCDFTB) calculations, using the DFTB+ program package. The SCCDFTB approach uses an arranged set of integrals derived from the DFT calculations, which leads to a substantial reduction in the computational time. The SCCDFTB calculations have recently been shown to successfully describe the potential energy and mechanical properties of the carbon nanostructures, showing a good agreement with experiments and higherlevel theoretical methods [1517]. In this study, the SlaterKoster (SK) type parameter [18] was implemented, and the selfconsistent convergence criterion was set to be 10^{−6} a.u. For the geometry optimization, the structure was considered fully optimized when the forces on each atom to be converged were less than 10^{−5} a.u.
In this section, the Young's moduli of straight and curved graphenes with different curvature widths, W, (as shown in Fig. 1) was determined under axial strain based on the aforementioned SCCDFTB calculation. The axial strains are obtained by simulating axial tension, which is applied by changing the atomatom distance in small steps (0.01 Å) and then relaxing the model via the DFTB method. Four different curvature widths for graphenes are applied to obtain the deformation potential energy at different strains to consider the effect of magnitude of the curvature on the Young's moduli of the graphenes. Figure 2 illustrates the optimized schematics of the four different curved graphenes.
The diagrams of changing the potential energy versus the strain for straight and curved graphenes are shown in Fig. 3. As can be seen, the potential energy exactly behaves as the parabola diagrams. The average potential energy is achieved by dividing the potential energy by the total number of atoms in the graphene at the minimum potential energy.
Figure 1: The curved graphene with curvature width W
Figure 2: The different curvature width for graphenes
Figure 3: The deformation potential energy versus the strain for straight and curved graphene
From the Table 1, it is obvious that the average potential energy decreases slightly with decreasing the curvature width of graphene. For example, the average potential energy is equal to 44.85 eV for a curved graphenetypeIV and 44.77 eV for a curved graphenetypeI. Then, we have calculated the Young's modulus, Y, of the graphenes along the axial direction as a function of length to evaluate their hardness. The Young's modulus of the graphenes is calculated by the conventional definition, which involves the second derivative of the deformation potential energy with respect to the strain with the following formula [26]:
(1)
Where the function E is the deformation potential energy of the system, ε is the strain and V_{0} is the equilibrium volume. The results obtained from the Young’s modulus of graphenes for different curvature widths are listed in Table 1. According to these results, the Young’s modulus of straight graphene is equal to 1.31 TPa, which is in a good agreement with the previous studies on graphene with a similar length. For example, Van Lier et al. [14] use abinitio methods and find that the Young’s modulus of graphene is equal to 1.11 TPa. Also, Jing et al. [33] use Molecular Dynamics (MD) to calculate the Young’s modulus of graphene. They report that the young's modulus of graphene is equal to 1.03 TPa. Moreover, Ganji et al. [34] in 2015 calculate the Young's modulus of straight graphene by SCCDFTB and MD calculations and find that this property is equal to 1.42 and 1.41 TPa, respectively.
Table 1. The average potential energy and Young's modulus for graphene for different curvature width 

graphene 
Average potential energy (eV) 
Young's modulus (TPa) 
curved graphenetype I 
44.77 
1.26 
curved graphenetype II 
44.80 
1.28 
curved graphenetype III 
44.81 
1.28 
curved graphenetype IV 
44.85 
1.29 
straight 
45.52 
1.31 0.16 
Figure 4: The optimized structure of a twocurved graphene
In addition, we observe that the Young’s moduli of curved graphenes are decreased by decreasing the curvature width of graphenes slowly from 1.29 TPa for curved graphenetypeI. This decrease in the modulus can be due to a decrease in the average energy of the atoms in the curved graphenes.
Moreover, we have studied the effect of the number of curvature on the average potential energy and Young's modulus of graphenes. The optimized structure of twocurved graphene is illustrated in Fig. 4. It is found that the average potential energy and Young's modulus of twocurved graphene are equal to 44.47 eV and 1.23 TPa, respectively. Thus, according to the previous calculated results, the average potential energy and Young's modulus of graphenes decrease with increasing the number of curvature. In this section, we have considered the effects of the StoneWale (SW), oneatom vacancy (onevac) and twoatom vacancy (twovac) defects on the mechanical properties of curved graphenes. The optimized structures of the SW, one and twoatom vacancy defect curved graphene systems are depicted in Fig. 5. The value of Young's modulus for the SW system is 1.26 TPa, which is slightly smaller than that of the curved graphenesTypeIV (1.29 TPa). It is also found that with increasing the number of atom vacancy in the curved graphenes the Young's modulus decreases from 1.25 for onevac system to 1.21 TPa for twovac system.
Figure 5: The optimized structure of the SW, one and twoatom vacancy defect curved graphene
4. Conclusions
In current study, we used the SCCDFTB approach to calculate mechanical properties of straight and curved graphenes such as Young's modulus and potential energy with different curvature widths. From the results, it can be calculated that potential energy decreases slightly with decreasing the curvature width of graphene. Also, we found that the Young's modulus of straight graphene was equal to 1.31 TPa and this mechanical property decreased by decreasing the curvature width of graphene and Young's modulus of graphene with minimum of curvature width being equal to 1.26 Tpa. Moreover, we studied the number of curvature on mechanical properties and we showed that the graphene with two curvatures had Young's modulus and potential energy about 1.23 TPa and 4.47 eV respectively. Also, we considered the effects of oneatom vacancy and two –atom vacancy defect and found that Young's modulus decreased about 1% and 4% respectively in comparison with straight graphene.
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