4. Plasma fluorination

Lee and et al. used inductively coupled NH3 plasma with a power of 1 <sup>10</sup><sup>3</sup> W/m<sup>2</sup> at a pressure of 500 mTorr [40]. Samples were processed in the two plasma regions: the bulk plasma region (Rbulk) and the sheath region (Rsheath). In both regions, reduction and nitridation processes began immediately once the NH3 plasma was exposed to the GO films. Just like in Kim's work XPS measurements showed that in both cases a gradual increase N-pyrrolic and a decrease N-quoternary with an increase of treatment time of up to 30 min were observed. The authors also observed an increase in the ratio of N/C and a decrease of the O/C, which can be explained by the substitution of nitrogen compounds at the sites of oxygen functional groups on the r-GO films. At the same time, the electrical conductivity of the r-GO films in the bulk plasma region increased significantly after 10 min of treatment. On the other hand, the optical transmittance of the r-GO films in the Rbulk decreases gradually with increasing processing time, while for the sheath plasma region it first decreases, then after 5 min of processing starts to increase gradually. The observed effects are attributed to the fact that the reactions in each region were shown to be different. The authors explain the observed effects by the difference in the reactions in these regions. In the Rsheath, the physical reaction was dominant because of the accelerated ion bombardment by the strong electric field. In general, comparing these sample locations in the reaction chamber, the authors conclude that the reduction in the Rbulk is more

Mohai et al. in [42] estimated the penetration of nitrogen and argon ions into the GO using the stopping and range of ions in matter (SRIM-2013) program. Calculations have shown that at energies of 20–50 eV, the depth is equal for the two ions. Thus, plasma can only modify narrow

It was shown in [43] that, as a result of nitrogen plasma treatment, the defect formation causing an increase in the intensity of the Raman peak D of the spectra depends of location of the samples in the reaction chamber. The substrate was placed "face down" into the plasma chamber (Figure 1), which significantly reduced the formation of defects in the rGO. In the investigated rGO samples, the electrical resistance increased, it is possible that this is due to the

predominance of defect formation over doping and restoration of the graphene lattice.

Figure 1. Schematic view of plasma system used for nitridation of graphene oxide.

effective.

near-surface regions.

12 Graphene Oxide - Applications and Opportunities

Plasmas of carbon tetrafluoromethane (CF4) and sulfur hexafluoride (SF6) are more often used for the fluorination of GO by plasma [8, 52–54]. In [8], the effect of plasma treatment of SF6 and CF4 ions on GO in the structure of an organic solar cell was investigated by methods of Raman spectroscopy, XPS, ultraviolet (UV) and infrared (IR) spectroscopy, and photoelectric characteristics measurements. For this purpose, reactive ion etching in a plasma with a power of 20 W with a duration of 10 to 60 s at a pressure of 20 mTorr was used. The results of the research showed that the use of GO films functionalized in the SF6 and CF4 plasma makes it possible to increase the conversion efficiency of solar energy from 0.56 to 2.72%. And the best result was achieved by processing in plasma SF6. From measurements of Raman spectra it follows that the ratio of the Raman spectra of ID/IG spectra as a result of plasma fluoride treatment increases slightly (from 1.17 to 1.21). This means an increase in disturbances in the graphene lattice. The XPS method revealed the presence of two peaks of C1s and F1 s in the spectra. C1s peak authors are considered responsible for the formation of C-F and C-F2 bonds with energies at 288.7 and 290.9 eV after SF6 and CF4 plasma treatment. The peak F1 s shows the presence of two components of the C-F bond with energies of 685.4 and 688.1 eV corresponding to the semimetallic and semiconductor bonds. In this case, as the authors believe, fluorine should change the electrostatic potential on the outer surface. The large difference in the electronegativity between carbon (2.55) and fluorine (3.98), according to the authors, means that the C-F bonds on the surface are polar. This is equivalent to introducing a layer of dipoles over the entire surface, which can increase the electrostatic potential energy on the surface with covalent bonds [8].

in the opinion of the authors, that the structure of rGO was not damaged by plasma treatment. After the fluoridation of GO authors noted a weak p-doping accompanied by a decrease in mobility due to the appearance of new scattering sites associated with F-containing groups.

The study of the fluorination of various carbon materials by plasma makes it possible to understand more fully the effect of such treatment on the properties of graphene oxide. The effect of plasma SF6 fluorination on the properties of graphene was investigated in [55–59]. Baraket et al. the chemical vapor deposition (CVD) graphene was treated with a pulsed Ar/SF6 plasma (pulse duration 2 ms and period 20 ms, plasma exposure time was 6 s and the total treatment time was 60 s) at a pressure of 50 mTorr, where the reactive gas was 5% of the total flow rate [55]. The ion energy was on the average 3 eV, which the latter is lower than the bonding energy of C-C (3.6 eV) or C〓C bonds (6.35 eV) that form the graphitic plane of graphene. The total fluorine content after plasma treatment, measured by the XPS method, was 18 at.%. Moreover, no sulfur or sulfur compounds were observed after treatment. The intensity of the D band in Raman spectra caused by the disturbances increases significantly and exceeds the intensity G of the peak. Although the incident ion energies are low to cause impact-driven physical defects, the ion energies are sufficient to drive chemical reactions at the surface when reactive ions and/or neutrals are present during irradiation. Fluorine forms two types of bonds with carbon: ionic and semi-ionic bonds, which do not violate the planar

D band in the Raman spectra indicates an increasing amount of carbon bound to sp<sup>3</sup> in the structure of the modified grapheme [18, 33]. After heat treatment at T = 500�C in an Ar atmosphere with a duration of 10 min, the fluorine is completely removed, the intensities of D and D<sup>0</sup> peaks, as well as the ID/IG ratio drastically decrease [55]. In [56] the effect of pure SF6 plasma generated in an reactive ion etcher (RIE) system at a radio-frequency of 13.56 MHz on epitaxial grapheme. An rf power of 50 W and an SF6 partial pressure of 100 mTorr were used for all experiments. From XPS, ultraviolet photoelectron spectroscopy (UPS) and Raman spectroscopy of multilayer epitaxial graphene, the authors concluded that the configuration of sp<sup>2</sup> graphene remains unchanged after plasma treatment, and fluorination is limited to one or two surface layers. The authors believe that fluorination to the carbon atoms at the edges of graphite domains generated by ion-bombardment. Similar studies of the effect on graphene obtained by a micromechanical method were carried out by Yang [57]. The plasma processing conditions were as follows: 5 Pa of pressure, 5 W of power and a 5 sccm gas feed rate by different durations. From Raman spectra studies, the authors report that fluorination of singlelayer graphene occurs is much more feasible than that of bilayer and trilayer graphenes, for which high fluorination times are required. Studies have shown that the process of plasma fluorination of graphene is a reversible process. The annealing carried out by the authors at a

temperature of 970� K for 1 hour completely restored the original graphene structures.

monolayer graphene is more significant than its central part.

Chen et al. [58] from the Raman analysis of spectra of graphene exposed in plasma SF6 (5 Pa, 5 W, 5 sccm) concluded about p-doping of graphene. Moreover, the p-doping of the edges of

Zhang [59] used SF6 plasma to fluorinate the CVD graphene (37.5 mTorr, P = 5 W, at a gas flow rate of 2 sccm, DC bias voltage 13 V, the processing time was from 10 to 90 s). From the analysis


Plasma Treatment of Graphene Oxide http://dx.doi.org/10.5772/intechopen.77396 15

character of graphene, and covalent sp<sup>3</sup>

Zhou et al. used XPS, X-ray diffraction (XRD), AFM and transmission electron microscopy (TEM) methods to study GO obtained by the Hammers method [52]. The CF4 plasma treatment conditions were as follows: gas flow rate of 1.5 l/h, operating pressure of 20 Pa, a bias voltage of 200 V, the power of 240 W, and process duration of 1, 3, 5, 10, 15, 20 min, respectively. From XPS data it is shown that fluorination in plasma of CF4 leads to the formation of C-F, C-F2 and C-F3-bonds. Moreover, the intensity of the peak (C-F) increased at small times (up to 10 min) of treatment, and then sharply decreased. As the authors believe, this decrease may have resulted from the reduction in the conjugated π-domains. At the same time, the proportion of components CF2 and CF3 only increases with increasing processing time. On the other hand, the content of С-С and С-О decreases. The fluorination of GO can occur due to the substitution of hydrogen atoms in C-H and O-H bonds, as well as oxygen atoms in oxygencontaining groups. Using AFM and TEM, a thinning effect for GO exposed to plasma in CF4 was observed. The thickness of pristine GO after plasma exposes decreased from ~1.9 to 1.3 nm. The hydrophobicity of GO showed that with increasing treatment time to 20 min, super hydrophilic GO becomes neither hydrophilic nor hydrophobic [52].

In Ref. [53] studied the effect of an inductively coupled Ar/SF6 plasma with a power from 100 to 250 W at a pressure of 15–30 Pa on the property of graphene oxide. Using the X-ray energy dispersive spectroscopy, the fluorine content on the GO surface was found to increase depending on both the plasma power and the fluorination time. In this case, the ratio of the fluorine to oxygen concentrations (F/O) ratio content changes more rapidly than the change in fluorine to carbon concentrations (F/C). This can mean that fluorination is more due to the displacement of oxygen than carbon atoms. There is also a slight change in Raman spectra after plasma treatment lasting up to 30 min. Measurements of the volt-ampere characteristics showed a gradual increase in resistance with increasing processing time.

Yu et al. in [54] the high density plasma etcher was employed to carry out the fluorination at room temperature with the CF4 flow rate fixed at 20 sccm and the pressure kept at about 0.16 Pa. XPS measurements showed that after fluorination the C/F ratio increased from 17.2 to 27%, depending on the treatment time. The study of surface morphology by AFM showed that the structure of the graphene substrate is well preserved after the plasma treatment, implying negligible damage for the samples. These data are confirmed by the Raman spectra data, from which it follows that the ratio of the peak intensities D (at 1340 cm<sup>1</sup> ) G (at 1601 cm<sup>1</sup> ) and the ratio of these two peaks (ID/IG) all remain practically the same after fluorination. At the same time, unlike the work of [51], the thickness of the grapheme substrate increases from ~0.4 to ~1.0 nm, which the authors associate with fluorination. Perhaps this difference is due to the short processing times, which in this work were up to 20 s, while in Zhou's work—up to 20 min. Studies of electrical properties after treatments with a duration of up to 10 s showed that the current–voltage (IV) characteristics remained practically unchanged, which indicates, in the opinion of the authors, that the structure of rGO was not damaged by plasma treatment. After the fluoridation of GO authors noted a weak p-doping accompanied by a decrease in mobility due to the appearance of new scattering sites associated with F-containing groups.

believe, fluorine should change the electrostatic potential on the outer surface. The large difference in the electronegativity between carbon (2.55) and fluorine (3.98), according to the authors, means that the C-F bonds on the surface are polar. This is equivalent to introducing a layer of dipoles over the entire surface, which can increase the electrostatic potential energy on

Zhou et al. used XPS, X-ray diffraction (XRD), AFM and transmission electron microscopy (TEM) methods to study GO obtained by the Hammers method [52]. The CF4 plasma treatment conditions were as follows: gas flow rate of 1.5 l/h, operating pressure of 20 Pa, a bias voltage of 200 V, the power of 240 W, and process duration of 1, 3, 5, 10, 15, 20 min, respectively. From XPS data it is shown that fluorination in plasma of CF4 leads to the formation of C-F, C-F2 and C-F3-bonds. Moreover, the intensity of the peak (C-F) increased at small times (up to 10 min) of treatment, and then sharply decreased. As the authors believe, this decrease may have resulted from the reduction in the conjugated π-domains. At the same time, the proportion of components CF2 and CF3 only increases with increasing processing time. On the other hand, the content of С-С and С-О decreases. The fluorination of GO can occur due to the substitution of hydrogen atoms in C-H and O-H bonds, as well as oxygen atoms in oxygencontaining groups. Using AFM and TEM, a thinning effect for GO exposed to plasma in CF4 was observed. The thickness of pristine GO after plasma exposes decreased from ~1.9 to 1.3 nm. The hydrophobicity of GO showed that with increasing treatment time to 20 min,

In Ref. [53] studied the effect of an inductively coupled Ar/SF6 plasma with a power from 100 to 250 W at a pressure of 15–30 Pa on the property of graphene oxide. Using the X-ray energy dispersive spectroscopy, the fluorine content on the GO surface was found to increase depending on both the plasma power and the fluorination time. In this case, the ratio of the fluorine to oxygen concentrations (F/O) ratio content changes more rapidly than the change in fluorine to carbon concentrations (F/C). This can mean that fluorination is more due to the displacement of oxygen than carbon atoms. There is also a slight change in Raman spectra after plasma treatment lasting up to 30 min. Measurements of the volt-ampere characteristics

Yu et al. in [54] the high density plasma etcher was employed to carry out the fluorination at room temperature with the CF4 flow rate fixed at 20 sccm and the pressure kept at about 0.16 Pa. XPS measurements showed that after fluorination the C/F ratio increased from 17.2 to 27%, depending on the treatment time. The study of surface morphology by AFM showed that the structure of the graphene substrate is well preserved after the plasma treatment, implying negligible damage for the samples. These data are confirmed by the Raman spectra data, from

ratio of these two peaks (ID/IG) all remain practically the same after fluorination. At the same time, unlike the work of [51], the thickness of the grapheme substrate increases from ~0.4 to ~1.0 nm, which the authors associate with fluorination. Perhaps this difference is due to the short processing times, which in this work were up to 20 s, while in Zhou's work—up to 20 min. Studies of electrical properties after treatments with a duration of up to 10 s showed that the current–voltage (IV) characteristics remained practically unchanged, which indicates,

) G (at 1601 cm<sup>1</sup>

) and the

super hydrophilic GO becomes neither hydrophilic nor hydrophobic [52].

showed a gradual increase in resistance with increasing processing time.

which it follows that the ratio of the peak intensities D (at 1340 cm<sup>1</sup>

the surface with covalent bonds [8].

14 Graphene Oxide - Applications and Opportunities

The study of the fluorination of various carbon materials by plasma makes it possible to understand more fully the effect of such treatment on the properties of graphene oxide. The effect of plasma SF6 fluorination on the properties of graphene was investigated in [55–59]. Baraket et al. the chemical vapor deposition (CVD) graphene was treated with a pulsed Ar/SF6 plasma (pulse duration 2 ms and period 20 ms, plasma exposure time was 6 s and the total treatment time was 60 s) at a pressure of 50 mTorr, where the reactive gas was 5% of the total flow rate [55]. The ion energy was on the average 3 eV, which the latter is lower than the bonding energy of C-C (3.6 eV) or C〓C bonds (6.35 eV) that form the graphitic plane of graphene. The total fluorine content after plasma treatment, measured by the XPS method, was 18 at.%. Moreover, no sulfur or sulfur compounds were observed after treatment. The intensity of the D band in Raman spectra caused by the disturbances increases significantly and exceeds the intensity G of the peak. Although the incident ion energies are low to cause impact-driven physical defects, the ion energies are sufficient to drive chemical reactions at the surface when reactive ions and/or neutrals are present during irradiation. Fluorine forms two types of bonds with carbon: ionic and semi-ionic bonds, which do not violate the planar character of graphene, and covalent sp<sup>3</sup> -hybridized bonds. The increase in the intensity of the D band in the Raman spectra indicates an increasing amount of carbon bound to sp<sup>3</sup> in the structure of the modified grapheme [18, 33]. After heat treatment at T = 500�C in an Ar atmosphere with a duration of 10 min, the fluorine is completely removed, the intensities of D and D<sup>0</sup> peaks, as well as the ID/IG ratio drastically decrease [55]. In [56] the effect of pure SF6 plasma generated in an reactive ion etcher (RIE) system at a radio-frequency of 13.56 MHz on epitaxial grapheme. An rf power of 50 W and an SF6 partial pressure of 100 mTorr were used for all experiments. From XPS, ultraviolet photoelectron spectroscopy (UPS) and Raman spectroscopy of multilayer epitaxial graphene, the authors concluded that the configuration of sp<sup>2</sup> graphene remains unchanged after plasma treatment, and fluorination is limited to one or two surface layers. The authors believe that fluorination to the carbon atoms at the edges of graphite domains generated by ion-bombardment. Similar studies of the effect on graphene obtained by a micromechanical method were carried out by Yang [57]. The plasma processing conditions were as follows: 5 Pa of pressure, 5 W of power and a 5 sccm gas feed rate by different durations. From Raman spectra studies, the authors report that fluorination of singlelayer graphene occurs is much more feasible than that of bilayer and trilayer graphenes, for which high fluorination times are required. Studies have shown that the process of plasma fluorination of graphene is a reversible process. The annealing carried out by the authors at a temperature of 970� K for 1 hour completely restored the original graphene structures.

Chen et al. [58] from the Raman analysis of spectra of graphene exposed in plasma SF6 (5 Pa, 5 W, 5 sccm) concluded about p-doping of graphene. Moreover, the p-doping of the edges of monolayer graphene is more significant than its central part.

Zhang [59] used SF6 plasma to fluorinate the CVD graphene (37.5 mTorr, P = 5 W, at a gas flow rate of 2 sccm, DC bias voltage 13 V, the processing time was from 10 to 90 s). From the analysis of Raman spectra and XPS, the authors concluded that fluorination leads to covalent bonds and p-doping of graphene occurs. At the same time, the ratio ID/IG, which increased to ~ 2.9 after initial treatments of up to 50 s, starts to decrease continuously with further increase in the processing time. The authors explain this effect by the formation of a less stable fluorine group, which decays with increasing processing time.

that the fluoride desorption process is heterogeneous, due to the presence of various C-F bonding components, and can begin at temperatures of 150C with a low fluorine content [60]. Annealing of samples with a high content of fluorine, which has predominantly covalent components, showed that at T = 300C, the ratio of ID/IG intensities increases to 1.8, and then decreases to ~ 1.5 when annealed to 600C [60]. In [62], from the measurements of topography and currents obtained by the AFM method, fluorinated clusters sp3, with dimensions of ~ 20–30 nm, were detected.

Plasma Treatment of Graphene Oxide http://dx.doi.org/10.5772/intechopen.77396 17

An increase in the covalent bound fluorine content during plasma fluorination was observed on the surface of graphene nanoplates (GNPs), multi-wall carbon nanotubes (MWCNT) [72, 73] and highly ordered pyrolytic graphite (HOPG) [72]. Fluorination of GNPs and MWCNT in CF4 plasma leads to covalent bonding of fluorine with irregularities, defects, boundaries and with sp2 carbon atoms inside graphene sheets of both carbon materials [72]. Thus, the surface area of both materials is a parameter that partially determines the degree of functionalization. This is confirmed by the fact that surfaces of CNTs are easier to fluorinate than HOPG surfaces. This is the expected result, since the hybridized compound sp2 in graphite is much more stable

In reports of Tahara et al. [74] to suppress ion bombardments and improve the reaction with fluorine radicals on graphene, the substrate was placed "face down" in the plasma chamber. Graphene samples were investigated prepared by the mechanical exfoliation from graphite. For the fluorination process was used a RIE system. The graphene samples were exposed to Ar/F2 (90%/10%) plasma with a relatively low rf power of 5 W, a gas pressure of 0.1 Torr, and a total gas flow rate of 75 sccm at room temperature. The reaction time ranged from 0.5 to 30 min. The fluorination of graphene led to a sharp increase in the intensity of D-peak caused

intensity shown non-monotonic behavior and had the maximum at 3 min, while the 2D peak intensity decreased monotonously. As the authors believe, such behavior of the peak intensities indicates an increase of amount of fluorine atoms attaching to the graphene. The authors explain the decrease in the D-peak intensity by the competition between the defect-phonon scattering processes and the decrease in the lifetime of the electronic states, caused by an increase in the defect concentration. The paper also shows that the monolayer graphene was more reactive than bilayer. In addition, an annealed the fluorinated graphene samples in the atmosphere for 90 min at 573 K showed that fluorination in a plasma is a reversible process.

A review of the literature shows that plasma treatments in various gases are primarily used for the functionalization of graphene oxide to produce graphene oxide from graphene. Summarizing the effects of GO processing in various gases, the following conclusions can be drawn: 1. The effect of an oxygen-containing plasma leads to rapid etching of the GO layers, accompanied by the formation of a large number of defects. The etching rates depend on the type of plasma source used, the location of the samples in the reaction chamber and the distance

). At the same time, with increasing reaction time, the D peak

than the distorted sp<sup>2</sup> compound observed in nanotubes [21].

by the defects (1350 cm<sup>1</sup>

5. Conclusion

The results of the plasma fluorination of graphene in CF4 are reported in Refs. [26, 60–66]. Cheng et al. used the RIE system to treat CVD graphene CF4 with plasma (40 W, 700 mTorr) [67]. As a result of plasma processing in Raman spectra of graphene, after 10 s of processing, the intensities of the peaks (D, D', D + G), associated with the introduction of structural lattice disorderings [18, 33, 62], significantly increased. According to the authors, the reason for this can be the conversion of sp2 -carbon to sp3 -hybridization due to the adsorption of fluorine, which coincides with the conclusions of other authors [61, 62]. With an increase in processing time to 300 s, Raman peaks become almost invisible. This result is typical of strongly fluorinated graphene [68–70]. It was shown in report [60] that the ratio of the intensity D of the peak to the G peak increases significantly with initial treatments and then goes to saturation with a further increase in the fluorination time. At the same time, the ratio of I2D/IG intensities shows the opposite trend and reaches to saturation gradually. The intensity of the 2D Raman peak is related to two phonon doubleresonance Raman processes [33, 71]. The saturation yield can mean the absence of chemical etching by fluorine of carbon, if there is no ion bombardment [62]. At the same time, Shen and other [63] methods of optical microscopy and Raman spectroscopy observed thinning of graphene layers and an increase in structural disturbances during fluorination in CF4 plasma (at a power of 20 W at a pressure of 0.8 Torr), as well as in the report [52]. Exposure of graphene in a plasma with a duration of 5 s resulted in the removal of the upper layer of graphene. The paper notes that functionalization occurs due to the formation of covalent bonds that distort the lattice structures of graphene. As a result, the intensities of D and D 'peaks in the Raman spectra increase. It should be noted that different authors have no common opinion on this matter. From the analysis of XPS data and the results of measuring electrical characteristics, Cheng et al. in [60] state that at low fluorine content, ionic bonds of C-F components are introduced. With a high content of fluorine, covalent bonds dominate. The above increase in ID/IG and a decrease in the I2D/IG ratio in the Raman spectra, with short processing times (up to 10 s), was observed in [26, 61, 62] under close fluorination conditions in plasma. The Raman ID/IG peak ratio mapping images of fluorinated graphene showed that the flat portions of graphene are uniformly fluorinated [60]. While, in multilayered CVD graphene containing wrinkles, wrinkles, etc., heterogeneous fluorination occurs. As the authors argue, these areas are less susceptible to fluoridation. Similar results were obtained in [26, 61], who found that single-layer graphene is more efficiently fluorinated by plasma than two and three-layer graphene films. Measurements of the resistance of CF4 CVDgraphene fluorinated in plasma showed a significant increase in electrical resistance from several kΩ to several MΩ, [65] and even more than 100 GΩ [64]. Despite the high values of electrical resistance, there were regions with low resistances (such as bilayer islands, folds (2-layer height, width ~ 100 nm), wrinkles (line width < 50 nm), and ripples (fine parallel lines with spacing ~ 150 nm), which have small resistance [64]. In the authors' opinion, this is due to the weak fluorination of these regions. The increase in the resistance of fluorinated graphene is associated with the formation of covalent bonds [64, 65]. Annealing in a 30-min nitrogen atmosphere showed that the fluoride desorption process is heterogeneous, due to the presence of various C-F bonding components, and can begin at temperatures of 150C with a low fluorine content [60]. Annealing of samples with a high content of fluorine, which has predominantly covalent components, showed that at T = 300C, the ratio of ID/IG intensities increases to 1.8, and then decreases to ~ 1.5 when annealed to 600C [60]. In [62], from the measurements of topography and currents obtained by the AFM method, fluorinated clusters sp3, with dimensions of ~ 20–30 nm, were detected.

An increase in the covalent bound fluorine content during plasma fluorination was observed on the surface of graphene nanoplates (GNPs), multi-wall carbon nanotubes (MWCNT) [72, 73] and highly ordered pyrolytic graphite (HOPG) [72]. Fluorination of GNPs and MWCNT in CF4 plasma leads to covalent bonding of fluorine with irregularities, defects, boundaries and with sp2 carbon atoms inside graphene sheets of both carbon materials [72]. Thus, the surface area of both materials is a parameter that partially determines the degree of functionalization. This is confirmed by the fact that surfaces of CNTs are easier to fluorinate than HOPG surfaces. This is the expected result, since the hybridized compound sp2 in graphite is much more stable than the distorted sp<sup>2</sup> compound observed in nanotubes [21].

In reports of Tahara et al. [74] to suppress ion bombardments and improve the reaction with fluorine radicals on graphene, the substrate was placed "face down" in the plasma chamber. Graphene samples were investigated prepared by the mechanical exfoliation from graphite. For the fluorination process was used a RIE system. The graphene samples were exposed to Ar/F2 (90%/10%) plasma with a relatively low rf power of 5 W, a gas pressure of 0.1 Torr, and a total gas flow rate of 75 sccm at room temperature. The reaction time ranged from 0.5 to 30 min. The fluorination of graphene led to a sharp increase in the intensity of D-peak caused by the defects (1350 cm<sup>1</sup> ). At the same time, with increasing reaction time, the D peak intensity shown non-monotonic behavior and had the maximum at 3 min, while the 2D peak intensity decreased monotonously. As the authors believe, such behavior of the peak intensities indicates an increase of amount of fluorine atoms attaching to the graphene. The authors explain the decrease in the D-peak intensity by the competition between the defect-phonon scattering processes and the decrease in the lifetime of the electronic states, caused by an increase in the defect concentration. The paper also shows that the monolayer graphene was more reactive than bilayer. In addition, an annealed the fluorinated graphene samples in the atmosphere for 90 min at 573 K showed that fluorination in a plasma is a reversible process.
