5. Conclusion

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,

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

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


which decays with increasing processing time.

16 Graphene Oxide - Applications and Opportunities

of sp2


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 from the plasma, as well as the plasma power, gas pressure and processing temperature. The aggressive effect of oxygen plasma can be reduced by using 'gentle' treatments. In addition, plasma treatment in oxygen can be used to produce GO from graphene. The graphene oxide obtained in this way has a higher wettability of the surface, this circumstance is of interest in the development of biological sensors and gas sensors.

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The general effects of the action of plasma treatment can be reduced to the following points: the most noticeable functionalization of graphene oxide occurs at the initial processing times in plasma, duration, as a rule, up to 1 min. The effect of plasma on GO properties is limited to several atomic layers and does not affect the bulk properties of GO. When plasma treatments GO one of the primary tasks remains to protect the surface from the defect formation and etching of GO films.

In general, it should be noted that the effect of plasma treatments on GO properties is still poorly understood. If we compare the amount of work devoted to the plasma treatment of graphene and graphene oxide, then this amount for GO is much lower. A still poorly studied region remains the study of the effect of a plasma of a mixture of various gases on the properties of graphene oxide. Mechanisms of functionalization of graphene oxide in various plasma media have not been fully investigated. The problem of the "gentle" effect of plasma on the surface of graphene oxide has not been completely solved. When using plasma treatments to control the properties of graphene oxide, these problems must be solved in the future.
