Two‐Dimensional Fluorinated Graphene: Synthesis, Structures, Properties and Applications

Fluorinated graphene, an up‐rising member of the graphene family, combines a two‐dimensional layer‐structure, a wide bandgap, and high stability and attracts significant attention because of its unique nanostructure and carbon–fluorine bonds. Here, we give an extensive review of recent progress on synthetic methods and C–F bonding; additionally, we present the optical, electrical and electronic properties of fluorinated graphene and its electrochemical/biological applications. Fluorinated graphene exhibits various types of C–F bonds (covalent, semi‐ionic, and ionic bonds), tunable F/C ratios, and different configurations controlled by synthetic methods including direct fluorination and exfoliation methods. The relationship between the types/amounts of C–F bonds and specific properties, such as opened bandgap, high thermal and chemical stability, dispersibility, semiconducting/insulating nature, magnetic, self‐lubricating and mechanical properties and thermal conductivity, is discussed comprehensively. By optimizing the C–F bonding character and F/C ratios, fluorinated graphene can be utilized for energy conversion and storage devices, bioapplications, electrochemical sensors and amphiphobicity. Based on current progress, we propose potential problems of fluorinated graphene as well as the future challenge on the synthetic methods and C‐F bonding character. This review will provide guidance for controlling C–F bonds, developing fluorine‐related effects and promoting the application of fluorinated graphene.

from graphite, the ability to prepare graphene and its derivatives have triggered intense research in two-dimensional nanomaterials all over the world. [ 1 ] Subsequently, graphene-based materials receive much attention in nanotechnology because of their extraordinary properties, such as an ultrahigh theoretical specifi c surface area (2630 m 2 g −1 ), exceptional charge carrier mobility (200 000 cm 2 V −1 s −1 ), high thermal conductivity (≈5000 W m −1 K −1 ), high optical transmittance (≈97.7%). [ 2 ] Despite these aforementioned superiorities, pristine graphene suffers from several shortcomings including structural defects, chemical inertness and a zero bandgap. Thus, many functionalization methods such as chemical bonding, loading or generating functional groups or free radicals on graphene (or its derivatives) have been utilized to improve structural integrity, surface activity and processability. [ 3 ] The functionalization not only inherits unique carbon conjugated structures but also brings about a promise to alter the graphene's properties including dispersion, orientation, interaction and electronic properties. [ 4 ] Graphene oxide (GO) [ 5 ] and halogenated graphene (CX m , X = F, Cl, Br, or I), [ 4a , 6 ] typical members of graphene derivatives including fl uorographane [ 7 ] and thiofl uorographene, [ 8 ] have thousands of oxygen functional groups or halogen atoms on carbon nanosheets with the transition of carbon atoms from sp 2 to sp 3 hybridization. The chemical modifi cation endows graphene with many excellent properties, such as good dispersion in organic/water solvents, chemical activity on the surface via functional groups and tunable electronic properties, such as bandgap opening, charge transfer density and work functions. Despite numerous studies, GO and halogenated graphene continue to have several problems as follows: (1) GO has a variety of chemical bonds containing carboxyl, carbonyl, hydroxyl, lactone, and epoxide at graphene edge and basal-plan. The amount and concentration of these functional groups are not controlled. (2) Halogenated graphene usually is a mixture of nanosheets with different degrees of substitution and different halogencarbon bonds. (3) GO and halogenated graphene show less chemical and thermal stability than graphene as a result of a great amount of defects or substituents on the surface, especially for brominated-and iodine-doped graphene. The representative characteristics of halogenated graphene were shown in Table 1 . [ 9 ] Fluorinated graphene is regarded as the two-dimensional basic structural element of fl uorinated graphite fi rst synthesized by Ruff et al. in 1934. [ 10 ] Subsequently, fl uorinated graphite recevies much attention in self-cleaning, solid lubricants, superhydrophobic coating, and the electrode of electrochemical cell because of its extremely low surface energy, good chemical and thermal stabilities, and high electromotive force (4.57 V at 25 °C calculated by thermodynamic data) in lithium-fl uorinted graphite battery. [ 11 ] A typical method of preparing fl uorinated graphite is the fl uorination in fl uorinecontaining atmosphere, and the F/C ratios and the C-F bonds (covalent, semi-ionic or ionic) usually depend on the fl uorination conditions including the pressure, temperature and treatment time in fl uorine-containing atmosphere. Moreover, 2D fl uorinated graphene with single or few layers can be obtained by exfoliating fl uorinated graphite via mechanical or liquid phase exfoliation methods. High quality fl uorinated graphene offers a great potential for modulating various properties by controlling the microstructures (layer, size and surface chemistry).
Fluorinated graphene (CF x , x ≈ 0-1.12), which is a stable and wide-bandgap nanosheet in which a certain amount of C atoms is covalently bonded to F atoms, becomes a rising star of graphene derivatives because of its outstanding properties, such as a large negative magnetic resistance (a factor of 40 in a 9 T field), a wide optical bandgap (3.8 eV) and a high room-temperature resistance (>10 GΩ). [ 4a , 12 ] Fluorographene (fully fluorinated graphene, CF) is defined by Rahul R. Nair [ 4a ] as a carbon monofluoride of graphene with the F/C ratio of 1.0, which is also introduced or accepted by many groups. [ 13 ] Compared with other derivatives, fl uorinated graphene shows many unique properties because of the formation of various types of C-F bonds. First, because the F atoms has a higher electronegativity (4.0) than C (2.5), H (2.2), and O atoms (3.4), fl uorinated graphene show great potential for using as an atomically thin insulator or a tunnel barrier based on the heterostructure. [ 4a ] Second, because of the difference in electronegativity (1.5) between C and F atoms, fl uorinated graphene exhibits several C-F bonding characters from ionic, semi-ionic to covalent bonds controlled by the fl uorination conditions. [ 14 ] The C-F bonding character depends on the fl uorination levels according to theoretical calculation. [ 15 ] Third, fl uorinated graphene is regarded as an excellent cathode material for high-energy lithium batteries because of its ability to electrochemically store and release high-density energy (theoretical energy density of Li/CF 1.0 is 2162 Wh kg −1 ). [ 16 ] Thus, a Li/CF x battery shows high energy densities, good chemical stability, a long-term shelf life (>10 years) and minimal (<10%) self-discharge. [ 17 ] In particular, the theoretical specifi c capacity of Li/CF x ( x = 1) is 865 mAh g −1 with an average discharge potential between 4.5 and 5 V for a purely ionic C-F bond. [ 18 ] Fourth, covalent C-F bonds show a high response to biological signals because of the high orientation and polarity of the C-F bond. [ 19 ] Thus, fl uorinated graphene can be developed for various biological applications, such as promoting neuroinduction of stem cells [ 19b ] or as a single multimodal material for magnetic resonance imaging. [ 20 ] Finally, C-F bonds on the nanosheets greatly increase the hydrophobicity with an extreme low surface energy resulting in a super-hydrohophic or amphiphilic fi lm. [ 21 ] Recently, much progress has been made on the preparation and control of C-F bonding characters, F/C ratios (the F/C ratio is defi ned as the mole ratio of fl uorine to carbon) and confi gurations of fl uorinated graphene. The brief roadmap of a synthetic strategy of fl uorinated graphene is shown in Figure 1 . However, a comprehensive review about the relationship between C-F bonding character and various properties of fl uorinated graphene has not yet been reported. In this review, we present the recent progress and advances on synthesis methods, C-F bonding character, properties (bandgap, optical properties, stability, electronic conductivity, dispersibility, magnetic, tribological, mechanical (micromechanical) properties and thermal conductivity) and applications in energy conversion and storage devices, biological devices, quantum dots, supercapacitors and amphiphilic coating of fl uorinated graphene. This review provides guidance for regulating a variety of properties and performances of fl uorinated graphene based on designing and controlling its C-F bonding character, F/C ratios and confi guration. A strategy of structural design, potential problems and present/future challenges of fl uorinated graphene are also proposed.

Direct Gas-Fluorination
Fluorographene was prepared by Nair et al. [ 4a ] using XeF 2 gas to treat a graphene fi lm at 70 °C ( Figure 2 a). The fl uorination using XeF 2 gas is one of widely used technique to prepare fl uorographene with different fl uorinated structures becuase of mild and controllable process. The resultant fl uorographene showed high thermal stability up to 400 °C even in an atmospheric environment. Subsequently, the fl uorination process was investigated by Raman spectroscopy (Figure 2 b). An increase in the D band at 1350 cm −1 and a decreased 2D band at 2680 cm −1 indicated that the fl uorination degree of graphene increased with a long XeF 2 treatment time. Fluorographene was obtained until all D, 2D, and G bands disappeared. [ 4a ] Furthermore, fl uorographene was also prepared by fl uorinating graphene grown by chemical vapor deposition (CVD) on the Si substrate using XeF 2 gas at room temperature. [ 12a ] Fluorographene showed a dominant stoichiometry of C 1.0 F 1.0 and a high F/C ratio of graphene fi lm on both the front and back surface because of the effective etching on the Si substrate by XeF 2 gas. This effect was confi rmed by fl uorinated graphene (CF 0.25 ) on Cu foils because the Cu substrate cannot be etched by XeF 2 (Figure 2 c). Raman spectra showed the D, D' and D+D' peak,   [ 27 ] while G peak was broadened by the exposure to XeF 2 . This result indicated the introduction of a high degree of structural disorder in the fl uorinated graphene. [ 12a ] In addition, fl uorographene was also synthesized by treating graphene sheets in XeF 2 at 350 °C for 1 and 5 days in an inert atmosphere. [ 13a ] Despite a tunable F/C ratio, the large-scale production of fl uorographene is restricted by the high-temperature fl uorination and the expensive XeF 2 . Fluorine gas (F 2 ) is another important fl uorination agent to prepare fl uorinated graphene because of its high reactivity. Fluorinated graphene with different F/C ratios was synthesized by Wang et al. [ 22a ] GO was treated by F 2 at a low temperature (from room temperature to 180 °C) (Figure 2 d). The F/C ratios (0.65, 0.84, and 1.02) of fl uorinated graphene could be controlled by the concentration (2%, 5%, and 10%) of F 2 in a mixture of F 2 and N 2 gas. In addition, Cheng et al. [ 22c ] prepared fl uorinated graphene by the exfoliation of fl uorinated highly oriented pyrolitic graphite (HOPG), which was treated using 1 atm F 2 at a high temperature of 600 °C. The resultant few-layer fl uorinated graphene showed a high F/C ratio (CF 0.7 ). Interestingly, Sofer et al. [ 22e ] also presented an easy and weighable method for the fl uorination of Hummers GO and Staudenmaier GO in 20% F 2 /N 2 (v/v) at elevated temperatures and pressures. The high resolution XPS results indicated that the F/C ratio was 17.8% and 5.61% for Hummers GO and Staudenmaier GO, respectively.
Despite the high activity, the fl uorination of graphene using F 2 is limited by poor controllability of the C-F bonding characters (semi-ionic or ionic bonds) and F/C ratios, special equipment requirements and environmental hazards (high toxicity and corrosion). Thus, many other fl uorine-containing agents such as SF 6 , SF 4 or MoF 6 were also used for fl uorinating graphene. Pumera et al. [ 22d ] demonstrated the fl uorination of GO using SF 6 , SF 4 or MoF 6 . The surface elemental composition showed that GO synthesized by the Hummers method were thermally fl uorinated using SF 6 , SF 4 and MoF 6 at 800 °C with different F/C ratios of 1.92%, 0.53%, and 0.26%, respectively. Additionly, GO synthesized by Staudenmaier method were treated by microwave in SF 6 at 800 °C and 1000 °C and showed diffeirent F/C ratios of 4.25% and 0.49%, respectively. The results revealed that F/C ratios of GO could be tuned by different gaseous fl uorine-containing agents with the control of the temperature for the fl uorination. The structural changes of GO also led to the changes of F/C ratios under the treatment of SF 6 . [ 22d ] Despite recent progress, the fl uorination using XeF 2 , SF 6 , SF 4 or MoF 6 is still far from up-scale industrial production. . Reproduced with permission. [ 4a ] c) Optical changes of graphene upon single-side fl uorination. Reproduced with permission. [ 12a ] Copyright 2010, American Chemical Society. d) Scheme for preparing fl uorographene by direct-heating fl uorination of graphene-oxide. Reproduced with permission. [ 22a ] Copyright 2013, American Chemical Society.
Thus, exploring a low-toxic fl uorine-containing gas (or mixed gas) for mild, selective and high effi cient fl uorination is important for preparing various fl uorianted graphene in the future.

Plasma Fluorination
Compared with severe fl uorination of fl uorine-based gas, plasma fl uorination is considered to be an easy to control, mild and clean method for preparing fl uorinated graphene. During plasma fl uorination, the fl uorine radicals generated by the plasma technique adsorb onto graphene and form different C-F bonds. Recently, a variety of plasma sources, such as SF 6 , [ 23a,c,f,g,j ] CF 4 , [ 23b,d,e,i,k ] and F 2 , [ 23h ] have been used. Baraket et al. [ 23a ] synthesized fl uorinated graphene using electronbeam generated plasmas in Ar/SF 6 ( Figure 3 a), and found that C-F bonds in fl uorinated graphene could be reduced to original C-C bonds by removing F atoms via annealing (500 °C). Sherpa et al. [ 23g ] reported that the polarity of C-F bonds, depending on the C-F bonding characters (ionic, semi-ionic, or covalent) between F and C atoms, could be induced in fl uorinated epitaxial graphene using a SF 6 plasma-treatment in a reactive ion etcher system. They found that work function of fl uorinated graphene was controlled by the polarity of C-F bonds as well as by the degree of fl uorination. Recently, plasma fl uorination of graphene using SF 6 plasma was also investigated by Yang et al. [ 23j ] Interestingly, the fl uorination of singlelayer graphene is much more feasible than multi-layer because of large corrugations.
Bon et al. [ 23b ] reported the fl uorination of GO, obtained from thermally exfoliated graphite oxide, by the treatment of CF 4 plasma. C-F bonds in fl uorinated graphene could be changed to C-N bonds by reacting with butylamine (the nucleophilic reagent) at room temperature. [ 23b ] Yu et al. [ 23k ] also synthesized fl uorinated reduced graphene oxide (RGO) using CF 4 plasma at room temperature, and the F/C ratios (F/C ≈ 0. 17-0.27) were controlled by the plasma exposure time. Recently, Wang et al. [ 23i ] reported the fl uorination of CVD-grown single-layer graphene using CF 4 plasma. The results showed that F/C ratios of fl uorinated graphene were tuned by the conditions of the plasma; however, the resultant fl uorinated graphene and fl uorographene consisted of a mixture of CF x (x ≈ 1-3), and the spatial distribution of F on graphene was highly inhomogeneous. [ 23i ] K. I. Ho et al. [ 23e ] presented a one-step approach for the selective fl uorination of graphene using CF 4 plasma in a plasma-enhanced chemical vapor deposition (PECVD) system.
During the fl uorination, F-radicals preferentially fl uorinated graphene at a low temperature (<200 °C), while the defect was suppressed by screening out the effect of ion damage (Figure 3 c). When the fl uorination time increases, D peak in pristine graphene is remarkably intensifi ed and the G peak is broadened. Simultaneously, the D' peak originated from the intra-valley resonance of Raman Scattering, is obvious. [ 23e ] In addition to SF 6 and CF 4 , F 2 is also used for plasma fl uorination. Tahara et al. [ 23h ] developed a highly controlled fl uorination method of preparing fl uorinated graphene utilizing fl uorine radicals in Ar/F 2 plasma. To overcome ion attacks and facilitate the C=C addition reaction of graphene with fl uorine radicals, graphene was placed on the other side of the Si substrate to avoid direct contact with Ar/F 2 plasma (Figure 3 b).
High-density plasma is important for fl uorinating graphene with high F/C ratios because the fl uoride-containing ions (such as F − , CF 4 + , CF 3 + ) energies are lower than fl uoride radicals. Desipte a simple and effective method, the plasma fl uorination inevitably damages the carbon structure of graphene by severe ion bombardment at a relatively high temperature. [ 25a , 28 ] Furthermore, the production is limited because the preparation is highly restricted to plamas-treated area and expensive equipment. And ion damage during the plasma treatment is inevitable. Thus, the up-scale production of fl uorinated graphene via plasma fl uorination needs more developed technique and equipments.

Hydrothermal Fluorination
Hydrothermal or solvothermal fl uorination is another versatile method for preparing fl uorinated graphene and fl uorographene. The fl uorination effect depends on fl uorine precursors, such as hydrofl uoric acid (HF), [ 24c ] BF 3 -etherate [ 24b ] and diethylaminosulfur trifl uoride (DAST) [ 24a,e ] and hexafl uorophosphoric acid (HPF 6 ). [ 24d ] Wang et al. [ 24c ] presented a convenient method to fl uorinate dispersed GO using HF through a simple hydrothermal process. Note that some oxygen-containing groups were substituted by F atoms during the hydrothermal reaction. In addition, the F/C ratios were controlled by varying the temperature, times and HF concentration. Similarly, Gao et al. [ 24a ] reported the solvothermal fl uorination of GO fi lms through converting the oxygen-containing groups (mainly hydroxyl, epoxy, and carbonyl/carboxylic) to C-F bonds by treating GO with DAST in chloroform at 50 °C. More recently, Samanta et al. [ 24b ] prepared fl uorinated RGO with fl uorine coverage of 38 wt% using anhydrous BF 3 -etherate and alkyl thiol/alkyl amine on the gram scale.
GO is an excellent nanosheet for the hydrothermal fl uorianation because of many epoxide, hydroxyl, carboxylic and ketone functional groups on the surface The oxygen-containing groups can be removed or substituted by the formation of C-F bonds at high temperature using a suitable fl uorination solvent. Thus, the hydrothermal fl uorination shows great potential for fl uoriated graphene with high F/C ratios. Unfortunately, the uniform distribution of C-F bonds on fl uorinated graphene by the hydrothermal fl uorination has yet been reported.

Photochemical/Electrochemical Synthesis
Lee et al. [ 25a ] reported an environmentally friendly method of selectively fl uorinating single-side graphene using a solid fl uoropolymer CYTOP (Cytop, CTL-809) source and laser irradiation. The fl uoropolymer CYTOP decomposed under laser irradiation on the surface of a single-layer graphene fi lm on a SiO 2 /Si substrate. Active fl uorine radicals, generated by the decomposition of CYTOP, reacted with the sp 2 -hybridized carbon and formed C-F bonds (Figure 3 d). Gong et al. [ 25c ] prepared fl uorinated RGO by employing UV irradiation on GO dispersion in HF at room temperature. The synthesis of oxy-fl uorinated graphene via an electrochemical method was demonstrated by Bruna et al. [ 25b ] A graphite fl ake contacted a platinum wire as the working electrode was fl uorinated in HF (50 wt%) as the electrolyte. Despite an environmentally friendly method, the F/C ratios of fl uorinated graphene by photochemical fl uorination are relatively low, and the special fl uorination agents have yet to be developed.

Sonochemical Exfoliation
Sonochemical exfoliation of multilayer materials has been well researched because it is a versatile and nondestructive technique for preparing high-quality two-dimensional single-or few-layer nanomaterials. Solution-processed exfoliation has been employed for up-scale production of two-dimensional graphene and MoS 2 . [ 29 ] Single-layer fl uorinated graphene and fl uorographene were obtained by exfoliation from fl uorinated graphite assisted by ultrasonication. To date, many intercalated molecules have been used to exfoliate fl uorinated graphene including sulfolane, [ 9a ] ionic liquids, [ 26a ] surfactant, [ 26d ] N-methyl-2-pyrrolidone (NMP), [ 26b , 30 ] chloroform, [ 26g ] 2-isopropanol (IPA) [ 26e ] and acetonitrile. [ 26c ] The driving force of the intercalation can be evaluated by Gibbs free energy (Δ G ) of the intercalation compounding process triggered by the F atoms, which is defi ned in Equation ( 1) where Δ H and Δ S are the enthalpy and entropy for the intercalation of molecules or solvents respectively. Because of van der Waals attraction between two adjacent layers of fl uorinated graphite, Δ H is generally expected to be positive; resulting in a small and positive ΔG , and thus the exfoliation is mainly affected by T Δ S . At high temperature and pressure, the increase in Δ S leads to a decrease in Δ G , which indicates increasing driving forces. Thus, compared with graphite, fl uorinated graphene is easily exfoliated by the intercalation of molecules with relatively weak van der Waals attraction and a large interlayer space. High-yield single-or few-layer fl uorinated graphene with a specifi c F/C ratio is obtained. Zbořil et al. [ 9a ] prepared fl uorinated graphene (F/C = 1.00) by a single-step liquid-phase exfoliation. In this process, fl uorinated graphene was exfoliated from commercial fl uorinated graphite suspended in sulfolane at a 135 W ultrasonic bath for REVIEW 1 h at 50 °C. Chang et al. [ 26a ] reported an effective and low-cost exfoliation to obtain single and few-layer fl uorinated graphene (F/C = 0.25 or 0.50) in ionic liquid. In this method, ionic liquid (1-butyl-3-methylimidazolium bromide) intercalated into the interlayer of commercial fl uorinated graphite by mixing and incubating. After intercalation, black colloidal dispersion of fl uorinated graphene was obtained by ultrasonication. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) images revealed that two-dimensional fl uorinated graphene showed 1-5 layers with 2-10 µm in edge size. Among many organic solvents, NMP is considered to be an important intercalated molecule to exfoliate fl uorinated graphene because of its dipole moment value of 4.09 D ( Figure 4 a). [ 26b ] According to Gong's studies, the intercalation of NMP into the interlayer of fl uorinated graphite was accomplished by refl uxing for 2 h, and the subsequent exfoliation was facilitated by ultrasonication for 100 h. Feng et al. [ 26c ] reported a solvothermal exfoliation to prepare few-layer (1-3) fl uorographene with a high-yield production of 15%. The semi-ionic C-F bonds of fl uorographene exfoliated by chloroform (Figure 4 b) might be a result of hydrogen bonding during the intercalation. In addition, Wang et al. [ 26d ] prepared fl uorinated graphene by exfoliation of fl uorinated graphite using a cationic surfactant of cetyl-trimethyl-ammonium bromide (CTAB) and dopamine (DA). This intercalation was carried out at room temperature in air (Figure 4 c). Zhu et al. [ 26g ] demonstrated an easy method to synthesize fl uorinated graphene nanosheets by means of a one-pot sonochemical exfoliation of the commercially available graphite fl uoride powders in chloroform under ambient conditions without any additional pretreatments, assistant reagents, or special protections (Figure 4

d).
Liquid-phase exfoliation is a relatively simple method for high-quality fl uorinated graphene by optimizing the intercalation. The exfoliation at room temperature could reserve most original fl uorine atoms. The chemicals for the intercalation and the reaction condition (time, temperature and pressure) are signifi cantly important for the liquid-phase exfoliation. In general, polar molecules are more effective for the intercalation than the non-polar molecules. Besides, high temperature, long-term and high pressure facilitate the exfoliation for single-or fewlayer fl uorinated graphene after ultrasonication and separation. However, C-F bonds of fl uorinated graphene might be partially reduced during the high-temperature exfoliation, and importantly, the numer of layers is hardly controlled because of the weak selectivity of the exfoliation.

Modifi ed Hummer's Exfoliation
Hummer's method was widely used to prepare GO by the intercalation and oxidation of bulk graphite. Recently, the modifi ed Hummer's method attracted tremendous attention to the exfoliation of fl uorinated graphite because of its convenient, easy-control process. [ 20 , 21b , 31 ] Pulickel M. Ajayan et al. [ 21b , 31c ] developed a methodology to synthesize fl uorinated GO using a modifi ed Hummer's method. The magic-angle spinning (MAS) 13C NMR results revealed that there were two types of fl uorinated GO: partially fl uorinated GO (FGO) and highly fl uorinated GO (HFGO). FGO was hydrophilic similar to GO in hydrophilicity, while HFGO was relatively hydrophobic. Although the modifi ed Hummer's method improves the dispersion of fl uorinated GO in water or organic solvents by introducing numerous oxygen-containing groups, this severe reaction inevitably partially destroys C-F bonds of fl uorinated graphene.

Thermal Exfoliation
Fluorographene can also be exfoliated from fl uorinated graphite by thermal exfoliation. Dubois et al. [ 27 ] prepared fl uorographene by thermal exfoliation of fl uorinated HOPG prepared using F 2 . Fluorographene was obtained by fast elimination of interlaminar species of fl uorinated HOPG with a sharp increase in temperature accompanied by the color changing from greyish to black. [ 27 ] 3. Structures C-F bonding character including C-F bonds, F/C ratio, and confi guration largely determines the chemical (electrochemical), electrical, electronic, optical, magnetic structures, stability and hydrophobicity of fl uorinated graphene. Thus, the deep understanding of fl uoro-carbon structure is fundamental to control the properties and design the application of fl uorinated graphene. In this section, we discusse the C-F structural characteristics controlled by a variety of methods or technologies to offer a strategy for tuning C-F bonds precisely and uniformly.

C-F Bond
Chemical bonds are usually determined by the electronegativity between two bonding atoms. As a result, C-F bonds vary from covalent bonds, through semi-ionic bonds, to ionic bonds because of the extremely high electronegativity of fl uorine. This feature results in a more electrostatic character in the covalent C-F bond. [ 14b , 32 ] Sato et al. [ 33 ] experimentally confi rmed the existence of semi-ionic C-F bonds in fl uorine-graphite intercalation compounds. Recently, Lee et al. [ 14d ] synthesized fl uorinated graphene with semi-ionic bonds through a one-step liquid fl uorination using liquid ClF 3 as the fl uorine agent. Moreover, semi-ionic C-F bonds in fl uorine-graphite intercalation compounds and fl uorinated graphene were also reported based on theoretical calculations. [ 15,34 ] However, the length of semi-ionic and ionic C-F bonds has never been experimentally determined. [ 14a , 13b , 34,35 ] The semiempirical result is shown in Figure 5 .
The fl uorination of C-C bonds of graphene usually contains two competing reaction processes: (1) fl uorine radicals react with graphene to form covalent C-F bonds, in which the sp 3 -hybridized C atoms connect to F atoms and (2) fl uorine radicals react with graphene to form semi-ionic C-F bonds, in which the sp 2 -hybridized C atoms connect to F atoms. C-F bonds change from ionic to semi-ionic to covalent, accompanied by a decrease in F/C ratios by changing the fl uorination conditions (e.g., fl uorination agents, temperature and time). [ 14a ] Borini et al. [ 25b ] reported oxy-fl uorianted graphene with semiionic C-F bonds through the electrochemical intercalation of graphite in hydrofl uoric acid solution. Wang et al. [ 19b ] found that C-F bonds showed the partial transformation from semiionic nature to covalence with the increasing F/C ratios of fl uorinated grapehene, which was controlled by the exposure time in XeF 2 atmosphere. This transformation was also found in the liquid-phase exfoliation with the appropriate solvent such as chloroform. Feng et al. [ 26c ] found the partial transformation of covalent to semi-ionic C-F bonds in fl uorinated graphene exfoliated by chloroform due to the formation of C-H…F hydrogen bonds between chloroform molecules and F atoms of fl uorinated graphite. [ 22a , 33b , 36 ] Additionly, the low exfoliation temperature could contribute to the appearance of Csp 2 -F bonds. [ 33b ] Previous studies indicated that the partial transformation between ionic (semi-inoic) and covalent bonds could be caused by the interaction between C-F bonds and other molecules or materials. Importantly, the natrue of C-F bonds have a significant impact on the properties of fl uorinated graphene, such as work function, [ 23g ] reaction activity, [ 37 ] and electrochemical performance. [ 38 ] The presence and percentage of covalent, semi-ionic or ionic C-F bonds in fl uorinated graphene are investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). According to previous studies, the characteristic peaks of semi-ionic bonds between C and F atoms were observed at approximately 287-290 eV in the C1s XPS spectra (Figure 5 ), 685-688 eV in the F1s XPS spectra and 1050-1150 cm −1 in the FTIR spectra. [ 19b , 22a , 25c , 26c,f , 39 ] The characteristic peaks of C-F bonds in fl uorinated graphene are given in   Table 3 . Unfortunately, fl uorinated graphene containing ionic C-F bonds has seldom been reported.

F/C Ratio
Precise control of the F/C ratio of fl uorinated graphene is important for opening the bandgap, tuning electrical conductivity and optical transparency and understanding the structural transformation. Thus, beyond the aforementioned techniques in the second section, fl uorination conditions, including the reaction temperature, the species of fl uorination agents and catalysts, the type of carbon (e.g., graphene, GO, and RGO), the treated side and the sonochemical time, are utilized to tune the F/C ratios of fl uorinated graphene. [ 23e,k , 24c , 26b , 40 ] Yu et al. [ 23k ] reported that F/C ratios (0.17-0.27) of fl uorinated graphene were controlled by the time of CF 4 -plasma treatment. Similar results were also observed in a recent study by  In addition, Wang et al. [ 24c ] presented an easy, low-cost and effi cient hydrothermal-process to tune F/C ratios of fl uorinated graphene. The contents of each C-F-containing group (such as C-CF, C-CF 2 , and CF-CF 2 , CF, CF 2 , and CF 3 ) were dependent on the reaction temperature, time, and HF amount. An increase in the F/C ratios (from 0.11 to 0.48) was mainly attributed to the formation of the CF-CF 2 group. [ 24c ] Interestingly, Robinson et al. found that fl uorine saturation coverage differed when graphene fi lms were fl uorinated by XeF 2 on one or both sides. X-ray photoelectron spectroscopy and Raman spectroscopy revealed that fl uorine coverage saturates at 25% (C 4 F) for one-side fl uorination and at 100% (CF) for double-side fl uorination in XeF 2 at room temperature. [ 12a ] Gong et al. [ 26b ] also reported that the F/C ratios decreased with increasing ultrasonication time in NMP, which might be attributed to the increasing stretching vibration energy of C-F groups gained from the sonic power facilitating the departure of fl uorine. [ 26b ]

Confi guration
Fluorinated graphene and fl uorographene consisting of weakly bound stacked two-dimensional carbon monofl uorides are a basic building block of fl uorinated graphite. [ 4a , 12a , 41 ] To gain insight into C-F bonds, theoretical calculation on the confi guration of fl uorinated graphene is studied, such as chair, boat, stirrup, and twist-boat confi guration ( Figure 6 ). [ 12b ] The chair confi guration shows a two-dimensional alternate layer of F atoms and C atoms on both sides, whereas in a boat confi guration, F atoms alternate with C atoms in pairs. [ 12b , 42 ] In the stirrup confi guration, each C atom is bonded to an F atom in the way that consecutive fl uorine layers along a zigzag direction alternate with graphene layers, while the twist-boat confi guration derived from the boat confi guration has a slight twist to F atoms connecting two unique C atoms. [ 43 ] Different confi gurations of fl uorographene results in different properties including binding energy, chemical activity, stability, bandgap, Young's modulus and the lattice constant. [ 12b , 43a ] For example, fl uorinated graphene or fl uorographene with the chair confi guration has a lower theoretical binding energy than any other confi guration, and the stirrup confi guration is more stable than the boat and twist-boat confi gurations. [ 12b , 43a , 44 ] The fi rst-principles density functional theory (DFT) calculation showed that fl uorographene with the chair confi guration  had a direct bandgap of 3.1 eV, [ 45 ] which is in good agreement with the experimental data, while the calculated data (7.4 eV) based on the GW (where GW refers to the one-particle Green's function with the dynamic screened Coulomb interaction) approximation (7.4 eV) was twice as large as the experimental values. [ 4a , 12b , 43a ] There are different bandgap values between DFT and GW because GW full account of the quasiparticles and their interaction with light in fl uorographene included electron-hole (e-h) and electron-electron (e-e) interactions. [ 12b ] The high Young's modulus E of the chair confi guration was up to ≈228 N m −1 , which was twice the experimental value (100 ± 30 N m −1 ). [ 4a ] The difference between the calculation and experiment might be attributed to a large number of structural defects in fl uorographene because a certain portion of C atoms was not bonded to F atoms but formed dangling bonds. [ 43a ] However, compared with fl uorinated carbon nanotubes, the energy difference among various confi gurations of fl uorographene is very small, and this result indicates that fl uorographene is unlikely to be a pure single-crystal form in a chair, stirrup, boat, or twist-boat confi guration.

Properties
Fluorinated graphene shows many excellent properites such as wide bandgap of 3.1 eV, the highest theoretical specifi c capacity (865 mA h g −1 ), good thermal stability below 400 °C, distinct nonlinear feature and high hydrophobicity. In this section, we discuss a variety of properties including bandgap, absorption or luminescence, stability, electronic conductivity, dspersibility, magnetic properties, tribological properties, mechanical or micromechanical properties, and thermal conductivity. These properties are signifi cantly important for the application of fl uorinated graphene.

Band Gap
Graphene shows great potential for advanced electronic devices because of unique electronic properties, such as zero bandgap and high carrier mobility up to 200 000 cm 2 V −1 s −1 . [ 41,46 ] However, a zero band-gap, specifi cally valence (π) and conduction band (π*) touching at a Dirac point, lowers achievable on-off ratios for fi eld emission transistors based on a graphene semiconductor. [ 3b , 47 ] Thus, opening the bandgap is crucial for the design and fabrication of high-performance graphenebased electronic devices. Theoretically, fl uorographene shows a wide bandgap of 3.1 eV because of the transformation from the trigonal sp 2 orbital to the tetragonal sp 3 orbital. [ 4a , 9a , 12,48 ] This feature offers great potential for tuning the bandgap of fl uorographene with different C-F bonding characters. Robinson et al. [ 12a ] prepared fl uorinated graphene fi lms (on one side) with fl uorine coverage of 25% (C 4 F) using XeF 2 . The calculation indicated that the bandgap of fl uorinated graphene increases with an increasing F/C ratio because of the interaction between the p-orbital of F and the π-orbital of C. The formation of sp 3 bonds led to a large change in charge densities and scattering centers in the conduction band ( Figure 7 ). [ 12a ] The band gap of C 4 F is 2.93 eV according to the density of states calculations. Moreover, when graphene fi lms were fl uorinated on both sides, fl uorographene (C 1.0 F 1.0 ) showed a large bandgap of 3.07 eV. [ 12a ] Liu et al. [ 48 ] investigated the bandgap of fl uorinated graphene with different F/C ratios. The results indicated that the C-F bonds in low-fl uorine-coverage fl uorinated graphene (CF 0.031 , CF 0.056 , and CF 0.125 ) were polar covalent bonds because of the high electronegativity of F atoms, and thus, they exhibited a metallic behavior. This behavior could be changed by increasing F/C ratios. CF 0.25 and fl uorinated graphene (CF 1.0 ) had wide bandgaps of 2.92 eV and 3.13 eV, respectively, according to the generalized gradient approximation (GGA) calculations. Interestingly, fl uorinated graphene (CF 0.5 ) in which C atoms bonded to F atoms on one side also exhibited metallic behavior ascribed to the exchange splitting of the dangling C-p z orbital with a coupling with an impurity state induced by F atoms. The results indicate that the bandgap of fl uorinated graphene is greatly infl uenced by F/C ratios. [ 48 ] Based on the density-functional GGA calculation, the bandgap can also be controlled by different confi gurations and layers of fl uorinated graphene and fl uorographene. Specifi cally, a chair confi guration shows a bandgap of 3.10 eV, while the bandgaps of the stirrup, boat, and twist-boat confi guration are 3.58, 3.28, and 3.05 eV, respectively. [ 12b ] In the chair confi guration, F atoms are alternately distributed on the plane. One F atom locates above the carbon layer, while the other one is under the same  layer. Thus, the chair confi guration has more symmetry than the stirrup confi guration. In addition, the stirrup confi guration is more signifi cant for the conduction state because charge density follows the chain characteristic. [ 12b ] Li et al. [ 49 ] calculated the bandgap of C 4 F with different layers by means of DFT computation. The results implied that bi-layer fl uorinated graphene (C 4 F) had a much narrower indirect bandgap than that of monolayer fl uorinated graphene. Additionally, the bandgap of C 4 F nanosheets was further decreased by increasing the number of stacked layers because the conversion from insulator to semiconductor based on the dipole-dipole interaction between two C 4 F layers induce a subtle interlayer polarization. [ 49 ]

Optical Properties
Fluorine-substitution on carbon atom dramatically changes the optical properties of graphene including the absorption band, photoluminescence and transparency. Robinson et al. [ 12a ] found that graphene fi lm was optically transparent in the visible region after treatment by XeF 2 . The absorption coeffi cient decreased after fl uorination by SF 6 [ 23c ] and CF 4 plasma, [ 23e ] which was in agreement with other fl uorinated carbon materials. [ 14b , 50 ] Recently, the absorption spectra of fl uorinated graphene with different F/C ratios has been studied to appreciate the effect of fl uorination on optical properties. [ 4a , 24e ] Nair et al. [ 4a ] investigated the optical transparency of fl uorinated graphene by fl uorinating in XeF 2 at 70 °C ( Figure 8 a). Graphene shows a peak at 4.6 eV and an absorption edge at ≈2.5 eV, which was in good agreement with a pronounced van Hove singularity, and was no longer linear above 2.5 eV (Figure 8 a). [ 4a , 51 ] However, the absorption spectra were drastically changed after the fl uorination. Compared with graphene, fl uorinated graphene showed low-intensity absorption with a weak and broad band in the range of 4.0 to 5.0 eV. It exhibited high transparency in the whole range because of the impurity scattering. [ 4a ] Furthermore, fl uorographene only absorbed light with energy >3.0 eV (blue range) (Figure 8 a). This result indicated that fl uorographene was nearly transparent in the range of visible light with the wide bandgap ≥3.0 eV. [ 4a ] Zhao et al. [ 24e ] reported that fl uorinated GO dispersed in CH 3 CN, synthesized by hydrothermal method with different reaction medium, exhibited two absorption peaks at approximately 220 nm and 250-350 nm, which were assigned to the π-π* transition of conjugated polyenetype structures in the carbon nanosheets [ 31b,c , 52 ] and a couple of conjugated aromatic domains with different sizes, [ 30 ] respectively. Gong et al. [ 39b ] found that the π-π* transition peak of GO red-shifted from 230 to 260 nm after fl uorination (Figure 8 b) because of an increase in the π-electron concentration and structural ordering based on the restoration of sp 2 carbon and the possible rearrangement of atoms. [ 26b , 53 ] Recently, the studies on the photoluminescence (PL) of fl uorinated graphene have attracted attention because it not only yields insight into understanding electronic properties but is also crucial for advanced semiconductor devices and energy harvesting. Jeon et al. [ 13a ] reported the room-temperature PL spectra of graphene and fl uorinated graphene dispersed in acetone using 290 nm (4.275 eV) excitation (Figure 8 d). The results showed that fl uorinated graphene (fl uorination for 5 days) exhibited two emission peaks at approximately 3.80 eV and 3.65 eV indicating wide bandgaps, while no emission was obtained in graphene with zero bandgap. [ 13a , 54 ] Specifi cally, the peak at 3.80 eV corresponded to the band-to-band recombination of a free electron and a hole, which was found in the bandgap of fl uorinated graphene measured by near edge X-ray absorption spectroscopy (NEXAFS) (Figure 8 c). [ 13a ] The peak at 3.65 eV was 156 meV (1260 cm −1 ) below the bandgap because of phonon-assisted radiative recombination across the bandgap where the C-F vibration mode was excited when the electronhole pair recombined. Analogously, two accompanying peaks at 2.88 eV and 2.73 eV were also observed in low-degree fl uorinated graphene (fl uorination for 1 day) (Figure 8 d). [ 13a ] Based on unique PL, fl uorographene can be developed for fabricating fl exible near ultraviolet LEDs by optimizing quantum yield.
Optical properties of ground-state fl uorinated graphene were also predicted by theoretical calculation based on DFT. [ 55 ] However, the calculation typically does not exactly match with experimental optical spectra because it does not take into account the interaction between two quasiparticles. [ 56 ] In this respect, the Bethe-Salpeter equation (GW-BSE) represents a more precise method than DFT for calculating the direct transitions because it takes into account electron-electron (e-e) and electronhole (e-h) interaction. [154][155][156] Samarakoon et al. [ 12b ] reported the in-plane absorption spectra of graphene and fl uorographene calculated by GW-BSE along with the random phase approximation (RPA) and GW-RPA, respectively. RPA was regarded as www.MaterialsViews.com www.advancedscience.com Adv. Sci. 2016, 3, 1500413   Figure 7. a) Calculated binding energy per F atom compared to the F 2 gas state. b) Sketch of the calculated C 4 F confi guration for the 25% coverage from (a). c) Calculated total density of states of single-side fl uorinated graphene for several fl uorine coverages. Reproduced with permission. [ 12a ] Copyright 2010, American Chemical Society. the result of the DFT level. As shown in Figure 9 , graphene had many notable peaks around 10-12 eV as a result of strong electron-hole correlations along with the appearance of bounded excitons in the ultraviolet region, opening the path toward an excitonic Bose-Einstein condensate in graphene that was observed experimentally. [ 12b , 56,58 ] This feature was also obtained for fl uorographene. A distinctive peak around 9.8 eV of fl uorographene emerged in GW-BSE that was evidently connected to strong electron-hole coupling and was attributed to the transition from the near-gap valence bands to the minimum conduction band. [ 12b ] Theoretical calculation provides an insightful understanding of optical properties controlled by C-F bonds, and results will promote more experimental studies on quantitative and qualitative descriptions of the optical properties of fl uorinated graphene in the future.

Stability
Compared with the instability of GO [ 59 ] and easily decomposed graphene, [ 60 ] fl uorographene shows good chemical and thermal stability as a result of strong C-F bonding energy. [ 4a ] Raman spectroscopy ( Figure 10 a) was typically utilized to study the stability of fl uorinated graphene because it can provide a wealth of information about the structures of graphene-based materials. The stability of fl uorinated graphene with different F/C ratios at high temperature has been recently reported in several studies. [ 4a , 12a , 22c ] Fluorinated graphene with a low F/C ratio could be partially recovered to pristine graphene by a short annealing-time at temperatures <400 °C, which was refl ected by a continuous decrease in the D band. In contrast, fl uorographene shows a high stability below 400 °C . [ 4a , 12a , 22c ] The removal of both C and F atoms in fl uorographene was observed by a prolonged annealing time at high temperature (≈450 °C). [ 4a ] In addition, according to XPS data, fl uorinated graphene, prepared using XeF 2 gas on SiO 2 , Au, and Cu substrates, lost approximately 50-80% of the initial F/C ratios over 10 days until the F/C ratios were not changed. [ 61 ] The change in C-F bonds by annealing at different temperatures was also demonstrated by an increase in electrical conductivity (Figure 10 b). [ 4a ] No current could be detected when fl uorographene was annealed T A below 200 °C. Fluorographene became weakly conductive, and the  The solid curve is the absorption behavior expected for a 2D semiconductor with E g = 3 eV. Reproduced with permission. [ 4a ] b) UV-vis absorption spectra of GO (dispersed in water) and FGO (dispersed in a mixture of ethanol and NMP) just after sonication. Reproduced with permission. [ 39b ] Copyright 2014, Royal Society of Chemistry. c) NEXAFS spectra of pristine graphene and fl uorographene with two different contents of fl uorine. [ 13a ] d) Room temperature photoluminescence emission of the pristine graphene and fl uorographene dispersed in acetone using 290 nm (4.275 eV) excitation. Reproduced with permission. [ 13a ] Copyright 2011, American Chemical Society. effective resistivity ρ = V/I decreased to ≈1 GΩ at 350 °C. The results indicated that the thermal stability and chemical inertness of fl uorographene were similar to Tefl on. [ 4a , 13b ] In addition, fl uorographene showed a good chemical stability in many liquids such as water, acetone, and propanol, and under ambient conditions except for strong reductants. [ 4a , 9a ] It was found that fl uorinated graphene could be reduced by hydrazine, potassium iodide, ultraviolet irradiation and alkylamine compounds. [ 9a , 12a , 61,62 ] Robinson et al. [ 12a ] reported the low temperature chemical reduction of fl uorographene by hydrazine with the process of 4CF n + n N 2 H 4 → 4C + 4 n HF + 2 n N 2 . Radek et al. [ 9a ] provided a pathway for defl uorination using KI in DMF. In this process, fl uorinated graphene transformed to metastable graphene iodide, which quickly decomposed to graphene and iodine at just 150 °C: CF + KI → KF + [CI]; [CI] → C + 1/2I 2 . Additionally,  reported that ionic C-F bond was selectively reduced by acetone treatment at a low temperature with the equation More recently, new fl uorinated graphene derivatives were prepared by the covalent modifi cation of fl uorinated graphene. [ 7,8,63 ] Stine et al. [ 63a ] fl uorinated CVD-grown graphene sheets followed by covalent modifi cation with ethylenediamine. They found that the intensity of the F 1s peak was reduced by ≈90% whereas a large N 1s peak at 399.5 eV appeared because of the removal of the fl uorine. Urbanová et al. [ 8 ] synthesized thiofl uorographene through the covalent functionalization (nucleophilic substitution). The thiofl uorographene showed a small region where F atoms were substituted by -SH groups. Interestingly, the semiconducting properties of thiofl uorographene could be potentially regulated by tuning the SH/F ratios.   Figure 10. a) Raman spectra of graphene fl uorinated to various levels and then annealed at different T . A,B,C) Raman spectra for weakly, mode rately and highly fl uorinated graphene, respectively. b) Changes in fl uorographene's ρ induced by annealing and I-V characteristics for partially fl uorinated graphene obtained by reduction at 350 °C. The curves from fl attest to steepest were measured at T = 100, 150, 200, 250, and 300 K, respectively. Reproduced with permission. [ 4a ]

Electronic Conductivity
Single-layer graphene shows high electron mobility because of its sp 2 hybridized C atoms with a p z orbital forming a πconjugated bond. Fluorination is widely used to chemically tailor the electrical conducitivity because it enables the transition from metallic/semiconducting to an insulating nature controlled by different F/C ratios.
Fluorographene, the thinnest two-dimensional insulator, shows a distinct nonlinear feature of I-V curves. [ 4a , 22c , 23i ] Different from C-C bonds of graphene, every C atom in fl uorographene with sp 3 hybridization is bound to an F atom. Thus, fl uorographene is an insulator because of the disappearance of πconjugated bonds. Wang et al. [ 23i ] investigated the typical I-V characteristics of fl uorinated graphene prepared by CF 4 plasma. Fluorinated graphene showed a linear I-V curve with a short fl uorination time (<10 min) with a resistance <10 MΩ because a low amount of F and sp 3 C atoms were regarded as defects in an sp 2 hybridized C network. Thus, the π-conjugated network of graphene is preserved. However, this network was destroyed by a long fl uorination-time (>10 min) using CF 4 plasma. Fluorinated graphene underwent the transition from semiconductor to insulator, resulting in a nonlinear curve with a resistance >1 GΩ. Compared with graphene, the resistance of fl uorinated graphene showed a sharp increase by more than 7 orders of magnitude (from 10 kΩ to >100 GΩ) when the fl uorine coverage is a few tenths of a percent. [ 23i , 64 ] Consequently, fl uorinated graphene with a low F/C ratio showed a semiconducting behavior with the sp 2 hybridized carbon network. [ 65 ] Moreover, the insulating properties of multi-layer fl uorographene with an extremely thin thickness (5 nm) could not be changed at temperatures <400 °C, and its dielectric constant and breakdown electric fi eld (EBD) were ≈1.2 and above 10 MV cm −1 , respectively. [ 66 ]

Dispersibility
Because of the presence of C-F bonds, fl uorinated graphene and fl uorographene are highly hydrophobic and diffi cult to disperse or solubilize in most solvents because of its low surface free energy. [ 21b , 67 ] However, the dispersion of fl uorographene in solvents is crucially important for the solution-processed fabrication of devices or applications as precursors for electrodes and composites. [ 26b , 26e , 68 ] Previous studies reported that hydrophobic fl uorographene could not be dispersed in ethanol because it has no free p z orbitals to form pseudohydrogen bonds with the hydroxy group of ethanol. [ 13a ] The pseudohydrogen bond has been demonstrated to facilitate the dispersion of graphene with the π bond (abundant free p z orbitals) in ethanol. [ 13a , 26b , 69 ] Gong et al. [ 26b ] studied the dispersibility of fl uorinated graphene in a variety of organic solvents. It was found that fl uorinated graphene showed much better dispersion in solvents with a large closed conjugated system formed by p z orbitals such as phenylethylene (PS), NMP, and THF than others with nonhybridized p z orbitals. [ 26b ] Specifi cally, in a homogeneous solvent, a free p z orbital acted as an electron acceptor and formed pseudo-hydrogen bonds with (C n F) x -F groups, thus resulting in an increase in dispersion of fl uorinated graphene. [ 13a , 26b ] Recently, the dispersion of fl uorinated graphene in water was improved by fl uorosurfactants in which perfl uorinated units were adhered on the surface of fl uorinated graphene and cationic or anionic units provided static repulsion. [ 70 ] Furthermore, fl uorinated GO could be well dispersed in many organic solvents with nonhybridized p z orbitals, such as CH 3 CN, chloroform, and DMF. [ 24e ]

Magnetic Properties
Graphene obtained by sonochemical exfoliation of high-purity HOPG shows a strongly diamagnetic response and no sign of ferromagnetism over a wide range of temperature, T . [ 71 ] A weak sign of paramagnetism becomes noticeable only below 50 K attributed to the edge states and point defects. [ 71,72 ] Interestingly, the introduction of F atoms in graphene causes a dramatic change in magnetic properties due to the presence of C-F bonds. [ 20,73 ] Nair et al. [ 73a ] reported that the paramagnetism in the CF x samlpes with x increasing from 0.1 to 1 was described by the Brillouin function.
where z = gJµ B H/k B T , g was the g -factor, J was the angular momentum number, N was the number of spins and k B was the Boltzmann constant. The number of spins N increased monotonically with x up to ≈0.9, and then showed some decrease for fl uorographene. The maximum M achieved by the fl uorination of graphene was one order of magnitude higher than that achieved by irradiation. Unfortunately, the concentration of magnetic moments was only ≈0.1% of the maximum hypothetically possible magnetism of one moment per carbon atom because F adatoms have a strongly towards clustering. [ 73a ] Tang et al. [ 73b ] reported that small F clusters that could be preferably formed around the vacancies in RGO produced a lot of magnetic edge adatoms. And such fl uorinated RGO have a high magnetization of 0.83 emu g −1 , a high magnetic moment of 3.187 × 10 −3 µ B per carbon atom and a high effi ciency of 8.68 × 10 −3 µ B per F adatom. Recently, fl uorinated GO was used as an effi cient magnetic resonance imaging (MRI) contrast agent by Ajayan et al. [ 20 ] Tang et al. [ 74 ] found that the uneven double-side partially fl uorinated graphene with the ripple structure become magnetic, whereas wrinkle structure showed nonmagnetic. And they also demonstrated that the magnetic moments could be signifi cantly increased by external tensile strain. [ 74 ]

Tribological properties
Generally, graphene shows good tribological performance due to its high chemical inertness, extreme strength, easy shear capability on its densely packed and atomically smooth surface. [ 75 ] Tribological properties of graphene are further improved by fl uorination. [ 76 ] Fluorinated graphene is consider as one of important ultrathin solid lubricants or lubricant additive of lubricating oils, lubricating coatings and anti-wear composites becuase of its low friction coeffi cient and high durability. Specfi cally, fl uorine atoms bound on carbon structure enhances nanoscale friction and reduces the adhesion and the number of free electrons by developing few van der Waals contacts and wide band gaps. Thus, C-F bonding structure endows fl uorinated graphene with excellent tribological performance. [ 76c,d , 77 ] Carpick et al. [ 76d ] systematically measured the friction between AFM tips and fl uorinated graphene with different F/C ratios. This method was useful to illustrate the mechanism for the enhanced friction. They found that the enhanced friction was attributed to the signifi cantly increased corrugation of the interfacial potential due to the highly localized negative charge concentrated at fl uorine sites, consistent with the Prandtl-Tomlinson model. [ 76d ] Park et al. reported that nanoscale friction on the fl uorinated graphene was 6 times larger than that on pristine graphene, while the adhesion decreased somewhat becuase the attachment of F atom to the C atom enable the transition of graphene to the tetrahedral sp 3 confi guration. [ 76c , 77a ] Hou et al. [ 78 ] found that fl uorinated graphene remarkably improved the reliability of the base oil and prolonged the friction time. Besides, trbological properties of fl uorinated graphene are also controlled by its microstructures (the arrangement of F atoms, corrugation and the number of atomic layers), F/C ratios, surface chemistry (species on the surface of fl uorinated graphene sheets). [ 76c,d , 77b , 78 ] Thus, many studies need to be presented to optimize trbological properties of fl uoroinated-graphene for ultrathin solid lubricant.

Mechanical or Micromechanical Properties
Fluorination usually affects mechnical properities of graphene such as Young's modulus ( E ) and intrinsic strength ( σ ) because of the presence of C-F bonds. Nair et al. measured the E and σ of fl uorographene using AFM. [ 4a ] Fluorographene exhibited a lower E (100 ± 30 N m −1 ) and a lower σ (≈15 N m −1 ) than of graphene ( E and σ of graphene are E = 340 ± 50 N m −1 and σ = 42 ± 4 N m −1 , respectively). [ 4a , 79 ] They speculated that the decrease in E and σ arised from longer sp 3 hybridized C-C bonds in fl uorographene than sp 3 hybridized C-C bonds in graphene. [ 4a ] Interestingly, the elastic deformation σ / E of fl uorgraphene showed litttle change in comparison of graphene because of the absence of structural defects during fl uorination. [ 4a ] The mechanism of controlling specifc mechanical properties including the strength, modules and deformation has yet been understood.

Thermal Conductivity
Graphene exhibits superior thermal conductivity because of effi cient phonon transfer in the 2D long-range sp 2 carbon framework by lattice vibrations. [ 80 ] To date, many highly thermal conductive graphene fi lm have been designed and prepared, and single-or few-layer graphene is widely used as thermal condutive nanofi ller in the polymer-based composite to increase thermal conduction. [ 80a,b,d , 81 ] Recently, fl uorinated graphene shows a great promise in combing heat dissipation and hydrophobic or self-lubricating properties. Huang et al. calculated theoretical thermal conductivity of fl uorinated graphene using non-equilibrium molecular dynamic (NEMD) simulations. [ 82 ] Results showed that thermal conductivity of fl uorinated graphene decreased during the fl uorination, and it increased when the F/C ratio approached 1.0. [ 82 ] They also found that thermal conductivity of fl uorinated graphene was less sensitive to strain than of graphene. This result might be attributed to that the phonon become less sensitive to tensile strain after fl uorination. [ 82,83 ] Despite great interest, improving thermal conductivity (diffusity) by exploring the key C-F structure is one of challenge for fl uorinated graphene.

Energy Conversion and Storage Devices
Fluorinated carbon materials (CF x ) were fi rst used as the cathode in lithium primary batteries by Watanabe et al. in 1972. [ 84 ] CF x was considered to be one of the ideal cathode materials for lithium primary batteries because of a variety of unique properties, such as high energy density, high average operating voltage, long shelf life, stable operation ability and wide operating temperature. Subsequently, Li/CF x batteries were fi rst commercialized by Matsushita Electric Co. in Japan in 1975. [ 85 ] Importantly, Li/CF x batteries have the highest theoretical specifi c capacity (865 mA h g −1 ; x = 1) in primary battery systems. [ 14c , 18c ] With an ultrathin two-dimensional layer-structure, fl uorinated graphene and fl uorographene are regarded as the most promising CF x to achieve the theoretical capacity because of their tunable F/C ratios and C-F bonding characters, favorable diffusion kinetics of lithium ions and large specifi c surface area. Recently, many studies focused on the performance of Li/CF x batteries using fl uorinated graphene or fl uorographene as the cathode material. [ 17b , 26c,e , 86 ] Feng et al. reported that lithium primary batteries using fl uorographene exhibited a remarkable discharge rate because of good Li + diffusion and charge mobility through nanosheets. [ 26c ] Fluorographene exfoliated by chloroform with semi-ionic F-C bonds showed a high specifi c capacity of 520 mA h g −1 and a voltage platform of 2.18 V at a current density of 1 C, accompanied by a maximum power density of 4038 W kg −1 at 3 C, which was almost four times higher than that of fl uorinated graphite ( Figure 11 a). [ 26c ] Moreover, fl uorographene showed an energy density of 1910 Wh/kg, which is higher than fl uorinated carbon nanotubes (≈1600-1800 Wh/kg). [ 87 ] Recently, they also prepared nitrogen and fl uorine co-doped graphene with superior reversible specifi c discharge capacity (1075 mA h g −1 at 100 mA g −1 ), excellent rate capabilities (305 mA h g −1 at 5 A g −1 ), and outstanding cycling stability (capacity retention of ≈95% at 5 A g −1 after 2000 cycles) as the anode material for lithium ion batteries. [ 88 ] Such results was attributed to the increased disorder and defects as well as the electrically conductive graphitic N and semi-ionic C-F bonds, and the highly wrinkled nanostructures caused by the co-doping of N and F. [  self-supporting fl uorinated graphene nanosheets by liquid exfoliation of fl uorinated graphite using IPA. [ 26e ] Fluorinated graphene not only had abundant fl uorine active sites for lithium storage but also facilitated the diffusion of lithium ions during charging and discharging. As a consequence, fl uorinated graphene exhibited a high reversible capacity of 780 mAh g −1 at 50 mA g −1 and excellent cycle performance for 50 cycles (Figure 11 b). [ 26e ] Rangasamy et al. fabricated a solid-state Li/CF x battery with a solid electrolyte of Li 3 PS 4 that had dual functions: the inert electrolyte at the anode and the active CF x component at the cathode. [ 17b ] The solid-state Li/CF x battery exhibited excellent capacity, good rate performance and a stable potential profi le with a capacity utilization of 1095 mAh g −1 beyond the theoretical capacity of a CF x cathode (when x = 1) (865 mAh g −1 ) (Figure 11 c). [ 17b ] In recently, Jeon et al. [ 89 ] reported that edgeselectively fl uorinated graphene nanoplatelets (FGnPs), which prepared by mechanochemically driven reaction between fl uorine gas (20 vol% in argon) and graphitic, demonstrated superb electrochemical performance with excellent stability/cycle life in lithium ion batteries. The FGnPs electrode showed an initial charge capacity of 650.3 mAh g −1 at 0.5 C and maintained a charge retention of 76.6% after 500 cycles. [ 89 ] Meanwhile, the FGnPs based dye-sensitized solar cells also displayed an outstanding performance (FF of 71.5%, J sc of 14.44 mA cm −2 and PCE of 10.01%) because of the high electronegativity of F atom ( χ = 3.98) and the strong C-F covalent bonds (C-F, 488 kJ mol −1 ) at the edges. [ 89 ] Results indicate that edge-selectively fl uorinated graphene is one of excellent materials for energy conversion and storage devices.
Xie et al. [ 90 ] fi rst reported a prototype of Mg/fl uorinated graphene battery with the capacity of 110 and 90 mAh g −1 at 10 or 50 mA g −1 , respectively. They utilized the fast surface redox process to replace sluggish lattice migration to improve the kinetics of Mg batteries resulting in good reversibility and rate performance. High performance benefi ts from the surface reaction at accessible fl uorinated functional groups of porous conductive frameworks. This proof-of-concept Mg/fl uorinated graphene system bypasses the sluggish diffusion of multivalent cations into the host lattice and the structure distortion at the cathode. Vizintin et al. [ 91 ] used fl uorinated RGO as an interlayer additive in lithium−sulfur (Li−S) batteries. Fluorinated RGO blocked the diffusion/migration of polysulfi des from the porous positive electrode to the metallic lithium electrode and thus prevented the redox shuttle effect. The results showed that fl uorinated RGO effectively improved the open circuit potential, cycling stability and capacitiy of Li-S batteries.

Bioapplications
Fluorinated graphene is of interest in many bioapplications because of its fascinating C-F bonds that enable biological responses [ 19b , 92 ] and paramagnetic behavior. [ 20 , 73c ] Loh et al. [ 19b ] used fl uorinated graphene as the scaffold for the growth of mesenchymal stem cells (MSCs) (Figure 11 d). In their study, fl uorinated graphene enhanced cell adhesion and proliferation of MSCs, exhibiting a neuro-inductive effect viaspontaneous cell polarization. Fluorinated graphene fi lms were   [ 20 ] www.MaterialsViews.com www.advancedscience.com Adv. Sci. 2016, 3, 1500413 highly supportive of the growth of MSCs, and C-F bonds had signifi cant effects on cell morphology and cytoskeletal and nuclear elongation of MSCs. [ 19b ] Moreover, the introduction of C-F bonds into GO caused a dramatic change in magnetic properties. Ajayan et al. [ 20 ] reported that fl uorinated GO was an outstanding carbon-based magnetic resonance imaging (MRI) contrast agent without magnetic nanoparticles (Figure 11 e). the results showed that fl uorinated GO could be potentially developed for a theranostic material with multimodal imaging, including MRI, ultrasound and photoacoustics, as well as the potential to pack hydrophobic therapeutic agents along the hydrophilic fl uorinated GO basal plane. [ 20 , 73c ]

Fluorinated Graphene Quantum Dots
Although fl uorinated graphene is a semiconductor with a wide bandgap and shows UV-fl uorescence, [ 12a , 13a , 64 ] the bundling sheets and low fl uoroescent intensity restrict the application in optoelectronic devices. [ 13a , 30,93 ] Fluorinated graphene quantum dots (F-GQDs) with the size <10 nm exhibits unique electronic and luminescent properties because of quantum confi nement and edge effects. [ 94 ] Tang et al. demonstrated that F-GQDs synthesized by cutting fl uorinated graphene through hydrothermal method, exhibited bright blue photoluminescence and upconversion properties. [ 93 ] Gong et al. [ 95 ] developed a activating-cutting strategy to obtain graphene fl uoroxide QDs with the tunable size and controllable fl uorine coverage. The graphene fl uoroxide QDs with good solubility and stability in water, display stable blue luminescence in hostile environment. This feature shows a great potential for the fabrication of advanced optical nanodevices. [ 95a ] Sun et al. [ 96 ] developed a new top-down method to simultaneously synthesize F-GQDs and GQDs by combining a microwave-assisted technique with the hydrothermal treatment. F-GQDs showed excellent photo-and pH stability in long-term and real-time cellular imaging. Results open a gate to the application of fl uorinated graphene in environmental engineering, solar cells, biological probes, bioimaging and energy technology. To date, the photoluminescence controlled by the defect and C-F bonding character is still unclear.

Other Applications
Fluorinated graphene can also be used for applications in supercapacitors, [ 24e ] electrochemistry [ 97 ] and amphiphobicity [ 21b , 98 ] applications based on unique properties controlled by F/C ratios and a two-dimensional layer-structure.
Zhao et al. [ 24e ] prepared solid supercapacitors using fl uorinated graphene as an electrode material. Cyclic voltammetry measurements showed that fl uorinated graphene prepared in dichloromethane exhibited the highest specifi c capacitance at 106.6 F g −1 , which was much better than GO. The results were also confi rmed by charge/discharge curves. [ 24e ] Pumera et al. [ 97 ] studied the electrochemical properties of fl uorographite with three different F/C ratios of 0.33, 0.47, and 0.75. The results revealed that the heterogeneous electron transfer was accelerated by increasing F/C ratios, and the fl uorographite with the F/C ratio of 0.75 showed the fastest rate of electron transfer ( obs 0 k ) at 2.69 × 10 −3 cm s −1 and 4.37 × 10 −3 cm s −1 in [Fe(CN) 6 ] 4-/3− and Eu 2+/3+ redox probes, respectively. And the overpotentials of ascorbic acid and uric acid oxidations decrease with the increasing F/C ratios. And the fl uorographite with F/C ratio of 0.75 provided a response to uric acid at 18.46 µA mM −1 , which more sensitive than that to ascorbic acid (2.15 µA mM −1 ). [ 97 ] C-F bonds drastically reduce the surface energy of graphene, resulting in a change in wetting behavior. Mathkar et al. [ 21b ] reported an amphiphobic coating of fl uorinated GO with a low surface tension of 59 dyn cm −1 , synthesized by oxidizing the basal plane of fl uorinated graphite. This method allows for unique, accessible, carbon-based amphiphobic coatings. [ 21b ]

Conclusion and Outlook
In this review, we have given an overview of synthetic methods, structures and properties of fl uorinated graphene that can be utilized for applications in high-energy storage, unique biological response and magnetic resonance imaging, fl uorinated graphene quantum dots, supercapacitors, electrochemistry and amphiphobicity. We have emphasized the importance and signifi cance of controlling C-F bonding characters, F/C ratios and confi gurations of fl uorinated graphene, fl uorographene and F-GQDs by fl uorination (gas or liquid phase) or exfoliation. The selective fl uorination enables graphene with different twodimensional confi gurations for various properties including wide bandgap, blue luminescence, excellent electrochemistry, high stability and self-lubricating. For example, an increase in the F/C ratio enlarges the bandgap of fl uorographene, while a low F/C ratio usually ensures charge transport based on π-conjugated structures. The covalent C-F bonds in gas fl uorination are crucial for thermal and chemical stability, while semi-ionic and ionic bonds endow fl uorinated graphene with a higher discharge potential for lithium batteries. Moreover, thermal conductivity, magnetic properties and luminescence of fl uorinated graphene are not well developed because of a complicated fl uoro-carbon structure.
Although signifi cant progress has been made, additional challenge for uniform up-scale synthesis/production, targetoriented fl uorination, the homogeneity of C-F bonding character, solution processability and removal of other fl uorides in applications must be addressed. It is very diffi cult to tune C-F bonding precisely at the specifc microstructure and/or chemical structure because of the strong fl uorination and the complicated chemical and microstructures (layer, size, defects, confi guration) of graphene. A versatile, low-cost and safe method of fl uorinating graphene has not yet been found, resulting in the limitation of a wide range of use in commercial applications. Furthermore, because the mechanism of the formation of different C-F bonding character is still unclear, fl uorinated graphene is often composed of a mixture of various types of covalent, semi-ionic and/or ionic C-F bonds with different ratios. As a result, fl uorinated graphene with different structures (layer, size, C-F bonds) needs to be separated and/or purifi ed before the use in advanced electronic devices. Thus, chemical methods or strategies for selectively fl uorinating graphene are of paramount importance.
To date, there is a huge number of opportunities and challenges for designing and synthesizing fl uorinated graphene with different structures such as core-shell, nanoporous spheres, nanocages and topologically nontrivial assemblies. The investigation of diffenent fl uorinated graphene will put insightful understanding of their properties. On the basis of controlling the selectivity of plasma-treatment, gas-and/or mild fl uorniation, the change in F bonding nature, bandgap or electronic interaction of fl uorinated graphene with the increasing F/C ratios and/or the specifi c C-F bonds will be investigated. This result will further illustrate the effect of C-F bonding character and confi guration on thermal conductivity, self-lubricating and optical properties.
In addition to controllability and uniformity, multifunctional fl uorinated graphene will be an interesting subject of intense and fruitful research in the future.A systematic study of thermal conductivity, magnetic property and luminscence will provide more special application in fl uorinated graphene and F-GQDs as well as other fl uorinated carbon materials. Besides, the electrochemical properties of fl uorinated graphene will be further improved to maxmize the energy density and power density by optimizing the microstructure, C-F bonding character, the interfacial wettability and cooperation with additives. By the integration of chemical groups, polymer chains and/or functional nanoparticles, fl uorinated graphene and its composites will show great potential for secondary batteries (e.g., Li, Na and Li-S battery), super-insulating materials, light emitting diodes (LED) and display materials. Based on much effort focused on the controllability of structures and optimized properties, in the future, fl uorinated graphene will exhibit excellent performance in fl exible nanoelectronics, energy conversion/ storage, special protective coatings, and tissue engineering.