Structural Stability and Electronic and Magnetic Properties of

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Structural Stability and Electronic and Magnetic Properties of Fluorinated Bilayer Graphene Chun-Hua Hu,† Peng Zhang,† Hui-Ying Liu,‡ Shun-Qing Wu,*,† Yong Yang,§ and Zi-Zhong Zhu*,†,∥ †

Department of Physics and Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China School of Science, Jimei University, Xiamen 361021, China § State Key Lab for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China ∥ Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen 361005, China ‡

ABSTRACT: As a first step toward gaining a microscopic understanding of the fluorination of bilayer graphene (BLG) as the electrode material for lithium-ion batteries, the structural stability and electronic and magnetic properties of fluorinated BLG have been investigated using density functional theory. For the fully fluorinated BLG (C1F), the chair configuration is found to be energetically more stable than the boat one, while both C1F’s exhibit semiconducting behavior due to the sp3hybridized C atoms. For the half-fluorinated BLG (C2F), two configurations, namely, C2F-1 and C2F-3, are energetically favored. The electronic characteristics of the half-fluorinated BLG depend sensitively on its atomic configurations. The C2F-1 exhibits a semiconducting nature, while C2F-3 is metallic. In addition, the presence of a fluorine vacancy defect in fluorinated BLG is also studied. It is manifested that the nonfluorinated C atoms (sp2-hybridized) in the fluorinated BLGs are of remarkable significance in improving their electrical conductivity. the fluorographene could inherit the mechanical strength of SLG, exhibit a large Young’s modulus of 100 N/m, and sustain strains of 15%. These prominent properties of fluorographene, together with its chemical inertness and thermal stability (remain stable up to 400 °C even in air), have promised it to be a good counterpart of Teflon. Recent progress in synthesizing or isolating multilayer graphene films9−11 has provided access to understand their physical properties and interesting transport phenomena, including the anomalous quantum Hall effect,12,13 the ballistic electron transport at room temperature14 and the novel manybody couplings.15 These effects resulted from the effectively massless Dirac fermion, the carriers derived from graphene’s valence bands which exhibit a linear dispersion degenerate near the so-called Dirac point. In addition, their outstanding structural characteristics, such as the high surface area, high chemical stability, and mechanical strength, have also led to their important applications in energy storage devices.16−19 The fascinating electrochemical properties of carbon fluorides, such as the thermal stability and excellent transport property, have attracted a particular concern on the fluorinated BLG. The physics of fluorinated BLG are rich and also important; however, to our best knowledge, few studies have been done on the fluorinated BLG. Available studies have been mostly

I. INTRODUCTION Recently, the fluorides have become more and more important in energy storage and conversion because of their great performance. They have already been introduced in many applications of energy conversion, such as electrolytes for fuel cells1 and transparent electrodes for solar cells.2 For the carbon fluorides (such as fluorinated graphite, carbon fiber, and coke, CxF or CFx), they have also manifested themselves to a great extent in the applications in high energy primary lithium batteries.3,4 The carbon fluorides as electrode material of lithium batteries were first demonstrated by Watanabe et al.5 in 1972 and then commercialized by Matsushita Electric Co. in Japan6 a few years later. The commercialized Li/CF1 (where C represents coke) batteries were able to exhibit many outstanding electrochemical properties such as high energy density (up to 560 W/kg), high average operating voltage (about 2.4 V vs Li+/Li), long shelf life (>10 years at room temperature), stable operation ability, and wide operating temperatures (from −40 to 170 °C). Recently, Yazami et al.7 reported that the energy and power densities are intimately dependent on the F:C molar ratio, and a Li/CFx (where C represents carbon fiber) battery with x = 0.76 shows the maximum power density of 8057 W/kg, which is about 14 times higher than the one of commercial graphite fluoride. These findings have caused renewed interests in the electrochemical properties of fluorinated ultrathin layers of graphite such as single layer graphene (SLG) and bilayer graphene (BLG). Representatively, Nair et al. reported their experimental findings on the fully fluorinated SLG (named fluorographene).8 They showed that © 2013 American Chemical Society

Received: October 18, 2012 Revised: January 22, 2013 Published: February 7, 2013 3572

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dedicated to the BLG’s electronic structures, such as its unique tunable band gap20−22 and compressible electron gas.23 In this work, besides the BLG’s electronic structures, the structural stability and magnetic properties of the distinctive structural characteristic of fluorinated BLGs are systematically investigated. Due to the fact that the composition of carbon fluorides (e.g., the fluorinated graphite and carbon fiber) experimentally obtained is always a mixture of C1F (fully fluorinated) and C2F (half-fluorinated BLGs),7,24 we have therefore focused our investigations on the structural stability and electronic and magnetic properties of only the fully and half-fluorinated BLGs. The importance of the present work can be further emphasized. First, fluorinated graphene is indeed of great importance in materials sciences, and so is the fluorinated BLG. The present work focuses mainly on the structural stability and the electronic and magnetic properties of the fluorinated BLGs. Second, conclusions obtained in this work can be very helpful in the microscopic understanding of the fluorinated carbon and its extent.

II. COMPUTATIONAL DETAILS Density functional theory (DFT) calculations were performed by using the projector augmented wave method (PAW)25,26 with the plane wave basis set, as implemented in the Vienna Ab initio Simulation Package (VASP). The exchange-correlation energy functional is treated within the local density approximation (LDA).27,28 The wave functions are expanded by the plane waves with a kinetic energy cutoff of 450 eV. The Brillouin zone integrations are approximated by using a special k-point sampling of the Monkhorst−Pack scheme.29 A series of Γ-centered k-point meshes of 15 × 15 × 1, 13 × 13 × 1, and 7 × 7 × 1 are employed for the pristine BLG, C1F, and C2F, respectively. A supercell model is used to simulate an isolated single BLG, where BLGs are supposed to be placed in the x−y plane. A vacuum layer of 20 Å in the Z-direction is chosen so that the interactions between the adjacent BLG are expected to be negligible. All of the atomic configurations are fully relaxed until the Hellmann−Feynman forces on all atoms are smaller than 0.01 eV/Å. Moreover, aiming at understanding the sensitivity of C1F and C2F to the spin-polarized effect, the spin-polarized calculations were first performed. It is shown that both the pristine C1F and C2F are insensitive to the spinpolarization effect; therefore, all of the results presented here are based on the nonspin-polarized scheme except for the cases of C1F and C2F with fluorine vacancy defects where DFT calculations are based on the spin-polarized schemes.

Figure 1. Atomic configurations and band structures of: (a), (c) Hexagonal BLG and (b), (d) Bernal BLG, respectively. In panel (b), C atoms in the upper and lower layers are shown in green and gray, respectively.

Table 1. Calculated Lattice Parameters, C−C Bond Lengths (dC−C), Interlayer Distances (dlayer), and Binding Energies (Eb) of the Hexagonal and Bernal BLG BLG

a (Å)

dC−C (Å)

dlayer (Å)

Eb (eV/atom)

Hexagonal Bernal

2.449 2.449

1.414 1.414

3.550 3.350

8.973 8.978

small binding energy difference. These calculated results of BLGs are in excellent agreement with available studies.22,30 On the other hand, both the Bernal and Hexagonal BLGs are semimetal, i.e., gapless semiconductor, and the difference between their band structures can be told from the corresponding two Dirac cones (see Figure 1(c) and 1(d)). The bands are nested for the Bernal BLG but interpenetrated for the Hexagonal one. In this study, because of the higher energetic stability and experimental accessibility20,31 of the Bernal BLG than the Hexagonal one, we will mainly focus our attention on the fluorination of BLG in the Bernal configuration. For the fully fluorinated BLG (C1F), two configurations, i.e., the chair- and boat-C1F, are considered and illustrated in Figure 2. The calculated structural and energetic properties are summarized in Table 2. The chair-C1F is calculated to be more stable than the boat-C1F by about 157 meV/C, and the two graphene layers in both C1F’s are separated by about 5.6 Å. This is well consistent with the case of graphite monofluoride, for which it is believed so far that the chair structure is more favorable.32−36 Moreover, in the chair-C1F, the C−F bond lengths are marginally larger, while the C−C bond lengths are distinctly smaller, than the corresponding ones in the boat-C1F (see Table 2), respectively. To understand the interatomic bonding characters of the fully fluorinated BLGs in both the chair and boat configurations (i.e., chair- and boat-C1F), the deformation charge densities of the specified planes of both the C1F’s are shown in Figure 3, where the range and interval value of deformation charge densities plotted are all the same. The deformation charge

III. RESULTS AND DISCUSSION Since BLG consists of two single atomic layers of graphene, two configurations (i.e., Hexagonal and Bernal BLGs) for BLG are considered and shown in Figure 1(a) and 1(b), respectively. The calculated lattice parameters, C−C bond lengths, equilibrium interlayer distances, and binding energies of both the BLGs are listed in Table 1, where the binding energy is defined as the total energy per atom of BLG with respect to the atomic energy of carbon. It is shown that the Bernal BLG (see Figure 1(b)) is marginally more stable than the hexagonal one by about 5 meV/C (see Figure 1(a)), and the equilibrium interlayer spacing of the Bernal BLG (3.35 Å) is a bit smaller than that of the hexagonal one (3.55 Å). Actually, interactions between two graphene layers in both the Hexagonal and Bernal BLGs are weak van der Waals forces, resulting in their very 3573

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Figure 2. Top and side views of (2 × 2) optimized configurations of the fully fluorinated BLGs in (a) chair and (b) boat configurations, respectively.

Table 2. Structural and Energetic Properties of the Fully and Half-Fluorinated BLGs, Including the Lattice Parameters, Inplane C−C Bond Lengths (dC−C), C−F Bond Lengths (dC−F), Interlayer Distances (dlayer), Buckled Heights (Δz), Band Gaps (Egap), and Binding Energies (Eb) C1F & C2F C1F chair-C1F boat-C1F C2F C2F-1 C2F-2 C2F-3 C2F-4 C2F-5

a/Å

b/Å

dC−C/Å

2.530 2.570

2.530 4.451

1.540 1.557a, 1.607b

2.552 2.511 4.955 4.972 4.963

2.552 2.511 4.955 4.972 4.963

1.538c, 1.556d 1.507 1.404−1.518 1.350−1.527 1.425−1.509

dlayer/Å

Δz/Å

Egap/eV

Eb/eV/C

1.365 1.362

5.592 5.604

0.489 0.624

2.787 2.705

12.588 12.431

1.361 1.400 1.389e, 1.386f 1.402 1.391

2.052 2.727 4.575 4.981 4.491

0.499  0.568  

4.037  metal  

10.773 10.001 10.657 10.542 10.344

dC−F/Å

a C−C bonds parallel to the x−y plane. bC−C bonds unparallel to the x−y plane. cInterlayer C−C bonds. dIntralayer C−C bonds. eC−F bonds in the interlayer spacing. fC−F bonds outside the interlayer spacing.

Table 3. Averaged Valence Electrons of Each F and C Atom Obtained by Bader Analysis, for the Chair-C1F, Boat-C1F, C2F-1, and C2F-3a fluorinated BLG C1F chair-C1F boat-C1F C2F C2F-1 C2F-3 a b

F1 (|e|)

F2 (|e|)

C1 (|e|)

C2 (|e|)

7.617 7.589

7.613 7.588

3.384 3.411

3.386 3.412

7.592 7.597

7.592 7.601

4.003b 3.403, 3.994b

3.405 3.398, 4.005b

F1 and F2 are fluorine atoms bonded with C1 and C2, respectively. Valence electrons of the nonfluorinated C atoms.

length in chair-C1F is smaller than that of boat-C1F (see dC−C in Table 2). On the other hand, the electronic total density of states (TDOS) and partial DOS (PDOS) of the chair- and boat-C1F are presented in Figure 4. The DOS below the valence band maximum (VBM) is divided into two explicit regions. A distinctive C−F (s,pz−s,pz) bonding is exhibited at around −25.0 eV for both C1F’s, corresponding to the strong ionic component of the C−F bond. The DOS from about −17.0 eV to VBM is dominated by the C−C sp3 bond, the C−F (pz−pz) σ, and C−F (px,py−px,py) π bonds, corresponding to the covalent component of the C−F bond. For the half-fluorinated BLGs, five candidate configurations (i.e., C2F-1, C2F-2, ..., C2F-5) are considered and illustrated in Figure 5(a)−(e). The first two candidates (i.e., C2F-1 and C2F2, see Figure 5(a) and (b)) represent the case where no F atoms are intercalated into the interlayer spacing of BLG, while the other three candidates (i.e., C2F-3, C2F-4, C2F-5, see Figure 5(c)−(e)) represent those where F atoms are intercalated into the two graphene layers of BLG. Their structural and energetic

Figure 3. Deformation charge densities of the (112̅0) and (100) planes for the (a) chair-C1F and (b) boat-C1F, respectively.

density is defined as Δρ(r)⃗ = ρ(r)⃗ − Σμρatom(r ⃗ − R⃗ μ), where ρ(r)⃗ represents the total charge density of C1F and Σμρatom(r ⃗ − R⃗ μ) is the superposition of atomic charge densities. From Figure 3, it suggests that the C−C bonds are mainly covalent, while the C−F bonds are ionic mixed with covalent in both the C1F’s. The C−C covalent bonds in the chair-C1F are a bit stronger than those in the boat-C1F, attributed to slightly shorter C−C bond lengths in the chair-C1F (see Table 2). Since the electron gained from the C atom by fluorine in the chair-C1F is about 0.03 |e| more than that in the boat-C1F, the ionic components of the C−F bonds in the chair-C1F should be a bit stronger than those in the boat-C1F. The averaged valence electrons of F atoms and their bonded C atoms in both C1F’s are summarized in Table 3. The stronger C−C bonds in chairC1F result in better structural stability and larger band gap of chair-C1F than those of boat-C1F because the C−C bond 3574

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Figure 4. Total and partial density of states of (a) chair-C1F and (b) boat-C1F.

Figure 5. (a)−(e) Five candidate configurations (C2F-1, C2F-2, ..., C2F-5) considered for the C2F. The F atoms adsorbed above the upper/lower layer of BLG are denoted by the red/blue solid cycles, while F atoms adsorbed below the upper/lower layer of BLG are denoted by red/blue “X”. (f)−(g) The top and side views of the optimized configurations of C2F-1 and C2F-3, respectively.

with fluorinated carbons and unfluorinated carbons. The influence of temperature on the structural stability could be significant; however, the present calculations are performed by first-principles technique based on the density-functional theory without considering the temperature factor. Moreover, the C− F bond lengths (1.361 Å) in the C2F-1 are almost equivalent to those in the C1F, matching also well with the case of graphite fluoride.37,40 On the other hand, for the C2F-3, the intralayer C−C bond lengths are not unique but range from 1.404 to 1.518 Å, where the bond lengths between the nonfluorinated C atoms (1.404 Å) are almost equivalent to those of the pristine BLG (see Table 1). While the C−F bond lengths in C2F-3 are found to be a bit larger than those in C2F-1. In addition, it is noticeable that the nonfluorinated C armchair chains are presented in C2F-3, which plays a critical role in its electrical conductivity as discussed below. Experimentally, the F/C ratio for graphite fluoride of the structure called C1F is often greater than one (1.1−1.3) and for the structure called C2F is greater than 0.5 (≈0.6). This fluorine excess is normally due to the fluorination of the graphene layer edge.37,41−43 However, the influence of the fluorine excess on the stability of the configurations of both systems C1F and C2F BLG is still unknown theoretically (before doing a detailed computation). To see the bonding characters in C2F-1 and C2F-3, the corresponding deformation charge densities of the (11̅00)

properties are summarized in Table 2, where the interlayer distance (dlayer) is the one between the averaged Z-coordinates of C atoms of each layer, and the buckled height (Δz) of each layer is the difference between the maximum and minimum Zcoordinates of C atoms in the layer. According to the binding energies listed in Table 2, the C2F1 is the energetically most favored configuration for the five models considered. The C2F-3 is the most stable structure for the case when F atoms are intercalated into graphene layers of BLG. In the C2F-1, the interlayer C−C bond lengths are just a little bit shorter than those in the graphene layer (see Table 2), while the energy required to break the interlayer C−C bonds (which is calculated to be about 4.1 eV) is so large that the C1F might be hardly obtained through further fluorination of C2F-1. This is consistent with the experimental results which show that graphite fluoride C2F can be formed by a direct fluorination of graphite between 340 and 360 °C,37−39 while C1F formed between 500 and 550 °C.37 Temperatures above 500 °C could greatly prevent the formation of the interlayer C−C bond of C2F-1; therefore, fully fluorinated BLG (C1F) could then be achieved by further fluorination of the half-fluorinated BLG (C2F-3). Consequently, C2F-3, which could only exist at high temperatures (>500 °C), can be considered as an intermediate phase in the formation of C1F. Thus, in the general case, all phases with F/C ratio between 0 and 1 have a similar structure, 3575

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than the other C−F bonds (see Figure 6(b)). The bonding characters of C−F bonds have been further verified by the Bader analysis (see Table 3). On the other hand, from the total and partial DOS plots for the C2F-3 shown in Figure 7(b), it suggests that the bands at around −25.0 eV are mainly contributed by the C(s, pz)−F(s, pz) bonding, while the bands from about −18.0 eV to the Fermi level are contributed by the C−C sp2, C−C sp3, and C(px, py, pz)−F(px, py, pz) bonding. Therefore, in the C2F-3, all the electrons are well bonded, and their states are below the Fermi level, except for the pz electron orbitals of the nonfluorinated C atoms. The DOS at the Fermi level is shown to be mostly contributed by the Cpz−Cpz π bonding between the nonfluorinated C atoms, which is also responsible for the metallic behavior of the C2F-3. Experimentally, the presence of the nonfluorinated C atoms in the carbon fluoride, even in a small amount, is shown to favor higher electron flow.7,24 Therefore, the sp2-hybridized nonfluorinated C atoms, but not the sp3-hybridized fluorinated C atoms (they are well bonded), are shown to be propitious for the conductivity of C2F. Whether or not the nonfluorinated C atoms are beneficial to the electrical conductivity of fluorinated BLG is closely dependent on their hybridizations. It is well-known that the defect states play significant roles in the semiconductor physics. What is more, defects (or impurity) are always there in the samples synthesized in many experiments, and we therefore further studied the fluorinated BLG with fluorine vacancy defects (Fvac). For the cases of chairC1F, boat-C1F, C2F-1, and C2F-3 systems, (3 × 3), (2 × 2), (3 × 3), and (2 × 2) supercells are employed to calculate the corresponding systems with Fvac, respectively. In these configurations, the smallest distances between the nearest neighboring Fvac are all larger than 5.1 Å. One Fvac corresponds to one more nonfluorinated C atom which is denoted as CF hereafter. The calculated spin-polarized total DOSs of the chairC1F, boat-C1F, C2F-1, and C2F-3 with Fvac are presented in Figure 8(a)−8(d), respectively. For the chair-C1F and boat-C1F with Fvac, the systems again exhibit semiconducting behavior; however, defect states are introduced in the band gaps. Moreover, both the chair- and boat-C1F’s are magnetic with the same magnetic moment of about 1.0 μB. The spin-polarized charges in both the chair- and boat-C1F’s with Fvac are mostly contributed by the pz electrons of the CF atoms. The induced magnetic moments in the two systems can be reasonably understood from the perspective of their structural and electronic characteristics. Since the pz electrons of CF atoms

planes are shown in Figure 6. The strong covalent bonds between C−C atoms in the C2F-1 are vividly illustrated by the

Figure 6. Deformation charge densities of the (11̅00) plane for the (a) C2F-1 and (b) C2F-3 configurations, respectively.

charge accumulation between the interlayer sp3 C−C atoms as well as the fluorinated and nonfluorinated C−C atoms (see Figure 6(a)). The valence electron obtained by the Bader analysis for the C1 atom (C1 and C2 represent carbon atoms of the two sublattices of graphene, respectively)44 is almost equal to 4.0 |e| (see Table 3). The bonding characters between C and F atoms are clearly ionic mixed with covalent, which is quite similar to the C−F bonds in the fully fluorinated BLG, C1F. On the other hand, from the total and partial density of states plots for the C2F-1 configurations as shown in Figure 7, the C(s, pz)−F(s, pz) bond in the C2F-1 contributes to the DOS peaks around the energy of −25.0 eV, while the bands (or DOS) from about −19.0 eV to the VBM are dominated by the sp3 C−C and C(px, py, pz)−F(px, py, pz) hybridization. Such a bonding character in the C2F-1 is again very similar to the case in the C1F. The C atoms in the C2F-1 are all in the sp3 hybridizations, together with the strong ionic and covalent characters of sp3 C−F, and such a bonding picture consequently leads to the large electronic band gap (>4 eV) of the C2F-1. For the C2F-3, the fluorinated C atoms are four-coordinated (with three C and one F atoms), while the nonfluorinated C atoms are three-coordinated (with three C atoms), leading to the coexistence of sp3 and sp2 C−C bonds in the C2F-3, as can be seen from Figure 6(b). Besides, there are two types of C−F bonds in the C2F-3, which are characterized by whether they are in the interlayer spacing or not. These C−F bonds are also mainly ionic but mixed with covalent, and the C−F bonds inside the graphene interlayer spacing are somewhat stronger

Figure 7. Total and partial density of states for (a) C2F-1 and (b) C2F-3 configurations, respectively. 3576

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Figure 8. Spin-polarized total DOS of the (a) chair-C1F, (b) boat-C1F, (c) C2F-1, and (d) C2F-3 with Fvac, respectively. The solid black lines represent the spin-polarized TDOS of the corresponding systems without Fvac. The Fermi level is set to 0 eV. Inset: three-dimensional plots of the spin-charge densities.

in both the C1F’s with Fvac are “unpaired” (i.e., being the dangling bond states), it is expected that the pz states of CF atoms will fall in the energy gaps, and the local characteristics of such pz electrons could induce magnetic moments easily in both the chair- and boat-C1F’s. For the half-fluorinated BLGs, in which half of the C atoms are nonfluorinated, it is noticeable that the C2F-1 with F vacancies introduces defect states at the Fermi level, as can be seen from Figure 8(c). Moreover, for the C2F-3 with F vacancies, more states are introduced in the band gap (around and above the Fermi level), as shown in Figure 8(d). From Figure 8, the C2F-1 and C2F-3 with Fvac are both magnetic while in different magnetic configurations. For C2F-1, all of the nonfluorinated C atoms are four-coordinated except for the CF atoms (created due to the presence of F vacancy) which are three-coordinated, and the spin-polarized charges around the Fermi level are then mostly contributed from the pz states of CF atoms, which could also lead to the induced ferromagnetic moment of about 1.0 μB (see also the inset of Figure 8(c)). However, for the C2F-3, in which all the nonfluorinated C atoms are three-coordinated, the spin-polarized charges of C2F3 with Fvac are contributed by the pz states of both the originally nonfluorinated C and the CF atoms (when F atoms are extracted, Figure 8(d)). Then, these spin-polarizations in the C2F-3 with Fvac are arranged in an antiferromagnetic way along the armchair C chains; namely, the majority and minority spin states are alternately arrayed in the two sublattices of the nonfluorinated C chains. Such a magnetic pattern can be understood from the distributed defect states over the sites of the sublattice along the nonfluorinated C chains and the exchange interactions in such systems. For Figure 8(d), not only could the nonfluorinated C atoms (those when F atoms are extracted) lead to magnetic moment but also these C atoms

could induce magnetism on the C atoms which are originally nonfluorinated. Furthermore, the bands (DOS) around/across the Fermi level in both the C1F’s and C2F’s with Fvac suggest that the existence of fluorine vacancies should be helpful for improving the electrical conductivities of the fluorinated BLG.

IV. CONCLUSIONS We have performed the first-principles calculations on the structural stabilities and the electronic and magnetic properties of the fluorinated bilayer graphenes. For the fully fluorinated BLGs, two configurations, i.e., the chair- and boat-C1F, are studied. It is shown that both C1F’s are semiconducting, with the chair-C1F energetically a bit more stable than the boat one by about 157 meV/C. All of the C atoms in both C1F’s are in sp3 hybridizations, resulting in their semiconducting properties. For the half-fluorinated BLGs, five candidate configurations are considered. Two energetically favorable configurations, i.e., C2F-1 and C2F-3, are found according to the calculated binding energies. The electronic characteristics of the half-fluorinated BLGs are shown to depend sensitively on their atomic configurations. The C2F-1 is presented to be semiconducting with a very large band gap of about 4.037 eV, which results from the coexistence of sp3 C−C and C−F bonds. However, the C2F-3 exhibits a metallic behavior with the electronic states at the Fermi level mainly contributed by the nonfluorinated C atoms (sp2-hybridized). Furthermore, the property of existence of fluorine vacancy defect in fluorinated BLG is also studied. It is manifested that the electrical conductivities of both the C1F’s and C2F’s could be improved when the fluorine vacancies are involved, due to the existence of nonfluorinated C atoms (sp2hybridized). 3577

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.Z. Zhu); [email protected] (S.Q. Wu). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National 973 Program of China (Grant No. 2011CB935903) and the National Natural Science Foundation of China under grant No. 21233004 and 11004165.

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