Specific values of fixed charge density Qf, calculated on the flat band voltage, determined from the capacitance-voltage characteristics, and the state density Dit at the interface with silicon, obtained from the difference between the middle-gap and flat band voltages.
Future electronics technology is expected to develop from rigid to flexible devices. This process requires breakthroughs in material properties, especially flexibility, in combination with desirable electrical insulating, semiconducting, or metallic properties. Graphene, being one of the recently developed two-dimensional (2D) materials, presents great promise as an active layer in a wide spectrum of electronics devices and, first of all, in field-effect transistors (FET). The development of optimized dielectrics for the graphene active layer is critical for graphene applications. The carrier transport in graphene films takes place at interfaces with dielectric or semiconductor substrates; therefore, the quality of such interface and the interaction of graphene films with nearby dielectric layers (charge carrier scattering) determine the device performance. Generally, the development of dielectric materials aiming at high performance device operation, proper mechanical properties, and low-temperature fabrication is not progressing well since the graphene thin film is very sensitive to surface conditions of dielectric layers. Solving the problem with dielectric layers in the case of nonorganic printed and flexible electronics is especially acute. As it is demonstrated in the present chapter, dielectric layers fabricated from fluorinated graphene suspension or in its combination with graphene oxide are the most promising for graphene-based flexible, printed, and transparent electronics.
- flexible electronics
- fluorinated graphene
- dielectric films
- leakage current
- resistive switching
- quantum dots
- negative differential resistance
Future technologies evolve toward flexible electronics (bendable, rollable, stretchable, or transparent) developed for a wide spectrum of bio- and medical applications, sensors and gadget displays, textile or clothing electronics, and so on [1–4]. Printing processes have attracted great attention, due to their compatibility with flexible substrates and materials, excellent prospects for applications, technical feasibility for scaling to large-area manufacturing and low cost. The two-dimensional (2D) printing technologies are at present used for creating intelligent components and smart systems of printed electronics, such as memories, displays, electronics, batteries, micromechanical systems, sensors, thin-film transistors, and other devices of modern electronics [1–4]. Flexible device technology does not support high temperatures, required for such materials as SiO2 or high-k Al2O3, HfO2, and ZrO2 . So, alternative 2D dielectric materials are highly demanded. Recent developments in technologies of liquid-phase graphene exfoliation lead to a significant progress in methods for creating 2D materials, including graphene, graphene oxide (GO), and other related materials (MoS2, WSe2, and so on) [6–10].
Graphene presents great promise as an active layer in a wide spectrum of devices of flexible electronics and, first of all, in field-effect transistors. Recent reports demonstrate successful realization of graphene FETs on flexible or even on stretchable substrates [8–13]. To make such applications possible, the development of optimized dielectrics for the active graphene layer is critical (gate and interlayer dielectrics, or/and supported layer for graphene). Nowadays, traditional high-k materials, such as Al2O3, HfO2, and ZrO2, are widely used for FETs. Nevertheless, the development of dielectric materials allows achieving high performance of the devices with excellent mechanical properties and low fabrication temperature that is in great demand.
Recently, graphene oxide (GO), one of the most well known graphene derivatives, has been exploited as a gate dielectric for graphene-based FETs [7, 8, 12, 13]. With its good mechanical and optical properties, this material offers a unique advantage for high performance flexible and transparent electronic devices since it can be formed on a graphene channel by solution-based or direct oxidation at room temperatures. Generally, GO has combined an excellent flexibility, a relatively large leakage current, and a strong limitation on enhanced temperature (even under the current flow). This limitation on enhanced temperature is connected with reducing GO: for instance, annealing at 100 °C typically leads to a decrease in GO layer resistivity by 4–5 orders of magnitude .
The most stable graphene derivative with dielectric properties is fluorinated graphene (FG), which is a low-k material with k = 1.2 and band gap of 3–8 eV [14–16]. Graphene can be fluorinated by a low-damaged CF4 plasma treatment  or by exposing the graphene to XeF2 gas to convert it to insulating fluorographene (C4F) . A new simple approach for graphene fluorination (treatment in aqueous solution of hydrofluoric acid) was recently suggested in Refs. [18, 19]. In the case of graphene suspension, such treatment leads not only to fluorination of flakes but also to additional flake fragmentation and exfoliation [19, 20]. As a result, a considerable decrease in thickness and lateral sizes of graphene flakes is accompanied by simultaneous transition of the flakes from conducting to insulating state. Smooth and uniform insulating films with low roughness can be created from the suspension on different substrates. The films from the fluorinated suspension are cheap, practically feasible, and easy to produce. The electrical and structural properties of the films from such fluorinated suspension with variable fluorination degree are discussed in the present chapter. Excellent characteristics of the dielectric film created from high fluorinated FG may be compared only with well developed SiO2 layers. In addition to excellent dielectric properties, reversible resistive switching effect and negative differential resistance (NDR) were detected in the films, created from the suspension with relatively low fluorination degree. Possible applications of these films are also considered.
2. Fluorination of graphene suspensions
The starting material was a graphene suspension that can be obtained using any of available approaches. It may be suspensions created by mechanical crushing of natural graphite, dimethylformamide (DMF) intercalation, ultrasonic treatment intended for splitting the intercalated particles, and centrifugation aiming at removal of nonsplit graphite particles . At the stage of intercalation, other organic solvents may be used, for instance, N-methyl-2-7 pyrrolidone (NMP) [21, 22]. Suspensions may be also created by electrochemical exfoliation [23, 24], liquid-phase exfoliation in water or water-based solution , or shear mixer [26, 27]. During suspension preparation, the natural graphite typically turned into flakes with characteristic size from 1 to several micrometers and thickness from 0.4 (monolayer) up to tens of nanometres. Parameters of flakes strongly depend on the technology used. After obtaining the suspension, it was subjected to a fluorination procedure [20, 28]. To this end, equal volumes of the suspension and a 5–10 % solution of hydrofluoric acid (HF) in water were mixed together. Fluorination time strongly increased with an increase in size and thickness of pristine graphene flakes and could be accelerated by enhancing temperature . Below, we demonstrate the fluorination process in the time scale when pristine graphene flakes have lateral sizes of 1–2 mm and thickness of 3–20 nm, without temperature acceleration. So, we are considering the admittedly inferior option which took sufficiently long time to clarify fluorination stages and all accompanying processes.
Let us consider here the reason for graphene fluorination during treatment in the aqueous solution of hydrofluoric acid (HF treatment). It is very important to note that practical difficulties associated with F reactions in two-dimensional (2D) carbon nanostructures are avoided if working with curved carbon nanosheets. An example of F reactions with corannulene molecules having a curved surface representing 1/3 of the structure of C60 fullerene was realized by dos Santos . Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10. Density functional theory (DFT) calculations of the HOMO−LUMO were used to demonstrate that gap variation for fluorinated corannulene, used as a prototype of C-based nanostructures, yields in values of band gap of 0.13–3.46 eV due to fluorination. Our experiments on the fluorination of CVD grown or mechanically exfoliated graphene in the aqueous solution of hydrofluoric acid also show corrugation of these materials during fluorination. In other words, the corrugation of domain boundaries is an initial driving force for subsequent F reaction with carbon atoms in 2D plain and subsequent corrugation [19, 30–32]. This type of reactions stimulated by deformations is also realized for the graphene suspension, and only thin (few layer) flakes are suitable for fluorination during HF treatment.
Liu et al.  have employed the first-principles method within the density functional theory to study the structural, electronic, and magnetic properties of the fluorinated graphene with different coverage of fluorine. The authors have found a strong variation of the graphene properties already at the early stage of fluorination (lower than CF0.25). Duan  has demonstrated that when fluorine bonds to a carbon atom, the latter one is pulled slightly above the graphene plane, creating what is referred to as a CF defect. These CF defects cause the graphene surface buckling. This corrugation was experimentally observed under fluorination in the aqueous solution of the hydrofluoric acid . Duan further showed that the addition of fluorine to graphene in some cases leads to the formation of an energy band gap near the Fermi level of 0.37–0.24 eV . Generally, it was demonstrated that the adsorption of fluorine on graphene surfaces is a promising approach to modify the properties of graphene, which may lead to more flexible electrooptical applications of graphene in the future.
To investigate the properties of FG suspension, some portions of the suspension were periodically used for the study and preparation of films. The substrates for films were silicon (Si) wafers. In depositing the films, the native oxide was removed from the surface of silicon by the hydrofluoric acid available in the solution. The deposited films were dried and rinsed with deionized water for removing the residual hydrofluoric acid and the organic component of the suspension, and then subjected to a second drying treatment for water removal. In other cases (especially for 2D printing), the solution in suspension was substituted for water. Due to hydrophilicity of FG flakes, the water-based suspension was stable.
3. The evolution of graphene suspension structure during fluorination
Fluorination of the DMF-based graphene suspension with a solution of hydrofluoric acid in water is considered in the present part. It has been found that, as a result of the interaction, the suspension particles undergo an additional splitting and size reduction by one-two orders of magnitude. Figure 1(a)–(d) shows the surface image of films created of Si substrate from the suspension after different times of fluorinated treatment. A schematic pattern for the action of the HF-containing solution on suspension particles resulting in additional exfoliation of graphene sheets, and formation of thinner and finer fluorinated graphene flakes has been proposed (Figure 1(e)). The driving force of these structural changes is found to be mechanical stresses, due to a difference in lattice constants and wettability of fluorographene [20, 28]. The similar fragmentation and exfoliation of graphene flakes are found for different kinds of graphene suspensions during the fluorination process.
Dependences of FG flake size on time of HF treatment are given in Figure 2. It is seen that the process of flakes fragmentation may be divided into several stages—up to 20 days, from 20 to 80 days, and over 80 days. These intervals correlate with changes in electrical properties of the films obtained from suspensions. About 20-day treatment was required for the film transition from conducting to insulating state. During fluorination for 20 to 80 days, the insulating properties of the films improved, and the current through the films and the charge density in the films decreased. As it will be demonstrated below, the most optimal property for the majority of applications is the fluorination within 60–80 days. The observed stages well agree with the behavior of leakage current through the FG film, which are also given below (Figure 4(a)).
Evidences proving the fluorination of suspension particles during the suspension treatment in a solution of HF in water were obtained by means of analysis techniques such as X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and Raman scattering spectroscopy (RSS) (Figure 3(a) and (b)) . Strong IR bands with maxima 1107, 1166, and 1230 cm−1 are clearly seen in the FTIR spectra. The observed modes supposedly correspond to the fluorine atoms, connected with sp3-hybridized carbon atoms. The studies of FTIR spectra with an increasing fluorination time have revealed that an adsorption band at 1112 cm−1 gradually changes into 1211 cm−1. The modes observed for the studied films are supposed to correspond to C-F bond and change depending on fluorination time. The gradual emergence of fluorinated graphene properties was traced in measured Raman spectra (Figure 3(b)) for the films, which was obtained from suspensions with different fluorination times. It is seen that an increased duration of the treatment results in a decreased intensity of peaks in Raman spectra. This effect is typical for fluorination of graphene and connected with band gap opening. The images of pristine graphene suspension and FG suspension also clarify the fluorination process. XPS spectra show a signal from fluorine F1s and carbon C1s, detected in the energy region of 687.7 and 284–286 eV, respectively (Figure 3(d)). The energy position and shape of the peaks are indicative of partial fluorination of suspension particles. Decomposition of C1s demonstrates peaks C-F with position at 288.3 eV and a peak C-CF at 286.5 eV. These XPS spectra correspond to fluorination degree ~ C4F and are typical for the case when suspension becomes transparent and transfers to a nonconductive state (see details in Section 6). In a longer HF treatment, it is possible to obtain higher fluorination degrees limited to the value ~ C2F. Due to this fact, we have a partially fluorinated suspension in all cases, and namely this FG suspension demonstrates many properties that are attractive for applications.
4. Insulating properties of hardly fluorinated graphene suspension
As it has been shown earlier, a considerable decrease in thickness and lateral sizes of graphene flakes (up to 1–5 monolayers in thickness and 20–30 nm in diameter) during fluorination is found to accompany the transition of the grapheme flakes from conducting to insulating state. The change in leakage current through the FG films as a function of the fluorinated time is given in Figure 4(a). One can compare these currents with leakage current of 17 mA/cm2 for 100 nm thick GO films  and ~20 A/cm2 for 4 nm GO films . So, the electrical and structural properties of the films suggest their use as insulating elements in thin-film nano- and microelectronics devices/structures. We have performed an analysis of the capacitance-voltage characteristics of metal (Ag, Au)/insulator (FG)/semiconductor (Si) (MIS) structures on different substrates. Dependence of relative dielectric constant ε on fluorinated time is shown in Figure 4(b). The dielectric constant of the films is determined to range from 1.1 to 3.2, depending on the fluorination degree of the material. It is important to mention that value of ε = 1.2 known for fluorographene [16, 20], appears in our films for relatively low fluorinated degree (between C4F and C2F).
The density of the fixed charge in the film Qf extracted from the flat band voltage of the capacitance-voltage characteristics and the density of interface states at the interface with silicon Dit are given in Table 1. Recall that the fluorinated graphene film was applied onto a silicon surface free of native oxide. The densities Qf and Dit proved to range from 6 × 1011 cm−2 to (0.3–0.5) × 1011 cm−2. With increasing duration of fluorination treatment, the densities Qf and Dit were found to decrease in value. The quality of obtained dielectric films was improved by increasing the treatment duration. On the whole, the obtained values proved to be much lower than the values of Qf and Dit, typical of many traditional dielectric coatings used in nano- and microelectronics such as Al2O3, HfO2, and ZrO2 . The typical values of built-in or interface charge density in the widely used Al2O3 films range within 1012–1013 cm−2 ; in HfO2, they vary within 1011–1012 cm−2 [36, 37], and in the Si/SiO2graphene/ZrO2 structures, they range within (1–15) × 1011 cm−2 eV−1 .
|Substrate||Time of HF treatment||20 days||40 days||60 days||80 days||150 days|
|Si||Qf, cm−2||(5–6) × 1011||1 × 1011||5 × 1010||(2–6) × 1010||(5–7) × 1010|
|Si||Dit, cm−2||(3–5) × 1011||5 × 1010||(1–3) × 1010||2 × 1010||(3–5) × 1010|
5. Relation between structural and electronic properties of FG films
Figure 5 shows SEM and AFM images of partially fluorinated films, created from the graphene suspension, treated in an aqueous solution of HF for 7 days (~CF0.10) and 20 days (CF0.23) . In both cases, the continuous films consisting of separate flakes may be observed in Figure 5. The bright corrugated areas of the film correspond to fluorinated regions, and the dark round regions correspond to nonfluorinated or weakly fluorinated graphene islands. The estimated flake sizes in suspension are 100–300 nm with a thickness of 0.5–5 nm for less fluorinated films (see Figure 2(a) and (b)), and 20–100 nm with a thickness of 0.5–2 nm for more fluorinated ones. The film thickness varies from 20 to 150 nm depending on the drop volume. Figure 5(a) and (b) presents the AFM image and the current map, measured with the probe for the same part of the weakly fluorinated FG film. Conductivity was found for graphene islands (dark areas), and bright areas (fluorinated part) were characterized by insulating properties. A comparison of AFM images clearly demonstrates that an increase in the HF treatment time leads to a decrease in the size of graphene islands and an increase in the size of the fluorinated areas between the graphene islands. So, generally, the films contain graphene islands and their properties were examined for pertinence to quantum dots (QDs).
The charge deep level transient spectroscopy (Q-DLTS) and transport measurements were used to characterize partially fluorinated films . It has been found that at the temperature range from 330 to 250 K, the current is described by equation I = Io exp(−E/kT), where E = 0.48 eV is the activation energy of the current flow through the film, and k is the Boltzmann constant. At lower temperatures, the current is close to be constant. The activationless current flow through the film is, most likely, associated with carrier migration (tunneling) through the traps in the FG band gap or graphene inclusions. Temperature dependence of the current is given in Figure 7, curve 1. Q-DLTS measurements demonstrate that only one type of activated process with energy 0.50 eV and density of localized states of ~ 1011 was observed. This activated process most likely corresponds to carriers passing over 0.5 eV potential barriers between graphene islands and fluorinated part of the film. It means that QDs with quantized electronic properties are not seen in these films. It may be connected with varying sizes of graphene islands and existing edge-related defects which provide the possibility of activationless transport in the films. It is worth mentioning that the arrays of QDs formed after similar fluorination of graphene or few layer graphene demonstrate the size quantization levels in Q-DLTS and transport measurements [31, 39–41]
6. Resistance switching effects in fluorinated films
Currently, the resistive memory approach is considered the most promising, as it allows obtaining a significantly lower time of memory operation (data overwriting). The most important parameters for the memory devices are a low overwriting voltage (lower than 3 V) and a high number of rewriting cycles and stability [42, 43]. The main problems of traditional metal oxide materials in the case of flexible electronics are relatively high fabrication temperature and very high cost. So, recent interest in the graphene-based materials results from its possible use for flexible resistive memory elements. Recently, some efforts have been made to create the resistive memory from graphene oxide (GO) or MoS2/graphene oxide composite [5, 10]. We have investigated fluorographene that is much more stable than GO graphene derivatives. The resistive switching effect for partially fluorinated graphene films (a two-phase system of graphene islands embedded in FG matrix) was observed for the first time .
The reversible resistive switching effect was found for the films, created from the suspension of partially fluorinated graphene for fluorination level of about ~ C4F (fluorination time of 40–60 days). Figure 6(a) and (b) shows current-voltage (I-V) characteristics for vertical Au/FG/Si/InGa structures and lateral Au/FG/Au structures, measured at two voltage sweeps. It is seen that the transition to the lower resistance of the layer occurs at voltage of about −1.5 V in vertical configuration and at about −4 V in lateral configuration. The reverse transition occurs by changing the polarity of the applied voltage. Repeated measurements prove the repeatability of such switching. The variation of temperature measurements from 80 to 350 K shows that these transitions may be observed in the entire studied temperature range. Daylight illumination is very important for carrier transport in fluorinated films (compare Figure 6(b) and (c)). The reset loops observed in dark conditions degrade under daylight illumination. The reason for this effect is connected with a strong decrease in the time of nonequilibrium carrier relaxation, directly observed in similar structures .
For a lower fluorination degree, the relation of resistances decreased, and for a higher fluorination degree, films became insulated.
One of the most important parameters of the material for resistive memory is the time of switching from high to low resistance states [5, 42, 43]. Q-DLTS measurements allow direct determination of the time of carrier emission (relaxation of nonequilibrium charge) from the specified traps . Time is one of the switching process parameters. The emission (relaxation) time of nonequilibrium carriers is determined from the position of the corresponding peak on Q-DLTS spectrum. It was found that after applying relatively high voltage (~4 V) we can fix the low resistive state of films for Q-DLTS measurements, and after 160°C annealing, the high resistive state can be restored.
The charge spectroscopy Q-DLTS and transport measurements were used to study traps in the films from partially fluorinated suspension in the states of both low and high resistance. The activation energy of traps E01 (0.50 eV), obtained from Q-DLTS method for the films in high resistive states, as mentioned above, may be interpreted as a potential barrier between graphene and fluorinated graphene areas, or between FG film and valence bands of silicon substrate. Transport and nonequilibrium recharging processes in the high resistance state were found to occur above all, due to carrier tunnelling through potential barriers in the films. Several types of traps for electrons and holes (one trap is demonstrated in Figure 7) with the density of 1010–1012 cm−2 were formed in the low resistive state of FG films. Among them are electron traps with activation energies of 0.15, 0.12, and 0.08 eV, and hole traps with energy of 0.34 eV. The minimum relaxation time of nonequilibrium carriers from different traps was found to be about 700 ns. The energy level position of corresponding traps from the conduction band of a silicon substrate equals 0.08 eV. The origin of the observed traps is supposed to correspond to traces of organic components, which are used during graphene suspension creation.
Figure 7(b) demonstrates temperature dependences of the current through the films, with thickness of 50 nm and 100 nm, at two electric field intensities. For the low resistance state (curve 2), the activation energy E for both films with different thickness is 0.09 eV, at the temperature range from 330 to 150–170 K . At lower temperatures, the current again is close to constant. The activationless current flow through the film is, most likely, associated with carrier tunneling on the traps in the FG band gap and graphene inclusions. Comparison of transport and Q-DLTS data demonstrates that the carrier transport in the low-resistance state is determined by the same traps (traps with activation energy 0.08 – 0.09 eV), and they form conductive channels in the films.
7. Films with negative differential resistance
Negative differential resistance (NDR) devices with nonohmic current-voltage characteristics are used in a wide range of applications, including frequency multipliers, memories, fast switches, high-frequency oscillators operating up to the THz range, etc., [44–46]. Currently, the theoretical predictions of NDR pertaining to graphene appear to prevail [47–49], and only a few experimental observations of NDR are presented in the literature (in fact, NDR is experimentally observed in lateral structures only in Ref. ). In the case of a two-barrier structure, it is possible to observe different regimes, from oscillation behavior in conductivity to only positive differential conductance . It is this situation that was observed in our experiments for FG films with a relatively low fluorination degree (F/C ratio) .
In our case for films created from suspensions, NDR was observed for relatively weakly fluorinated layers CFx with fluorination degree ranged from 0.10 < x < 0.23 (Figure 8(a) and (b)) . Two types of the film with slightly different flake size demonstrate similar I-V curves, but with different numbers of peaks. These films demonstrate one or few peaks in I-V curves, depending on the flake sizes, and that is explained by formation of barrier or multi-barrier systems. The current steps also seen in I-V curves are most likely connected with the presence of electrically active traps considered above. It’s worth noting that the obtained films were multibarrier systems due to the presence of a fluorinated network and non-fluorinated graphene islands. An increase in the fluorination degree first results in the increased number of the FG barriers. The origin of NDR for I-V curves shown in Figure 8(a) and (b) is mostly likely associated with the theoretically predicted gap in the transmission coefficient for carriers in the barrier between fluorinated graphene and graphene areas. It is caused by the competition of hole-to-electron transport and Klein tunnelling with resonant tunnelling in structures with potential barrier(s) .
The origin of the NDR was demonstrated to vary with an increase in the F/C ratio. Thick FG films (150 nm) with F/G ratio CF0.23 showed two peaks in the I-V curves, observed for voltages swept from negative to positive values (Figure 8(c) and (d)). The peaks were observed for the vertical configuration of measurements. A reverse voltage sweep produced I-V curves without any peaks (inset of Figure 8(d)). It is suggested that electrochemical oxidation-reduction reactions involving organic contaminants (traces of DMF, used to produce the graphene suspension) are located near defects, fluorinated carbons, or other special areas in our films. The temperature dependence of the conductivity at peaks exhibited activation behavior σ = σ0 exp(−Ei/kT), where Ei is the activation energy, k is the Boltzmann constant, and T is the temperature. Ei takes values 0.04 and 0.09 for peak 1, and 0.16 for peak 2.
The effect of NDR in fluorinated films widens the range of application including active device layers fabricated using 2D printed technologies on rigid and flexible substrates.
8. FG suspension for 2D inkjet printing technologies
The discovered control of the size of flakes in the graphene suspension during its fluorination serves to create inks for 2D inkjet printing [28, 51–53]. Suspensions of fluorinated graphene with nanometer size flakes are of interest for the development of 2D inkjet printing technologies, and production of thermally and chemically stable dielectric films for nanoelectronics . The printed fluorinated graphene films on silicon and flexible substrates have been demonstrated first time, and the charges in MIS structures have been estimated as ultra low values of (0.5–2) × 1010 cm−2.
The properties of the graphene oxide films may be greatly improved by adding the FG top layer. The thin FG and GO films are printed on silicon and flexible polyethylene terephthalate (PET) substrates (Figure 9 (a) and (b)). FG flakes have lateral sizes ranging within 20–100 nm and thickness of 0.4–1.5.nm. Figure 9 (a) contains the photo of printed graphene oxide lines with different number of layers. Layers in the left part of the substrate are coated by a layer of printed FG. The boundary between areas with and without FG is marked with arrows. The FG layers on the PET substrate are presented in Figure 9 (b). An AFM image of the FG layer on PET and the profile near the edge of FG film are demonstrated in Figure 9 (c) and (d).
The properties of two-layer films of FG on GO were the most extensively studied due to their revealed stability. Figure 9 (e) shows the current-voltage characteristics of the two-layer films of FG/GO, created by printing and dropping, measured in lateral configuration. It has been found out that the magnitude of the current through the two-layer structure is significantly lower than that through the separate films of GO or FG. The increase in the two-layer film thickness was shown to be not that significant, compared with the effect of reducing the current through the film by several orders of magnitude, especially in the case of printed layers. Applying the FG layer causes primarily the “healing” of structural defects in the GO film. The origin of this effect lies in good affinity between GO and FG, and in the electrostatic interaction between GO structural defects and FG flakes. Surface roughness of different films on rigid substrates was extracted from AFM images: the surface roughness for GO film was 8.1 nm, for FG/GO film, it was 5.7 nm, and for FG film, it was 1.5 nm. This effect is supposed to result from blocking of graphene oxide conductivity with small FG flakes. Significant decrease in FG/GO film roughness suggests formation of FG few layers on structural defects of GO films (local insulating island with thickness exceeding the average FG film thickness).
Assessment of thermal stability of different structures required investigation and comparison of the conductivity of GO and FG/GO films, obtained by applying drops and printing. Figure 9 (f) presents dependences of film resistance on temperature of isochronous annealing. The graphs show that the resistance of the two-layer structures, both printed and created by drops, changes not more than by 1–1.5 orders, while the change in the resistance of the graphene oxide films in both cases is 3–5 orders of magnitude. Therefore, the two-layer FG/GO films are much more stable than the GO films, since the application of the second layer greatly suppresses the GO recovery during heating.
9. Outlook and conclusion remarks
Creation of the fluorinated graphene suspension and inks are shown to extend the range of graphene-based materials from conductive to insulating and functional layers (Figure 10). The suggested approach for graphene or graphene suspension fluorination in the aqueous solution of hydrofluoric acid allows obtaining the partially fluorinated graphene with fluorine content lower than in CF0.5. This approach is cheap, practically feasible, and easy to produce. Moreover, films obtained from the partially fluorinated graphene may be conducting or insulating, depending on the suspension fluorination degree.
Fluorinated graphene cannot be structured using traditional technological approaches such as plasma treatment. This problem of the fluorinated graphene nanostructuring is successfully overcome by means of spin-coating or printed technologies. As a result, the use of FG suspension is the most convenient approach for a wide spectrum of applications, especially for flexible technologies.
The fluorination process in the case of graphene suspension causes additional flakes splitting and fragmentation. The small size of the fluorinated flakes makes them an excellent base for graphene-based inks. Dropped and printed films, obtained from the inks both on rigid and flexible substrates, demonstrate a great potential for a wide spectrum of electronic devices, especially for flexible electronics. Partial fluorination provides for FG films such effects as NDR and resistive switching that are promising for applications. The development of optimized dielectrics for the graphene active layer (active layer gate and interlayer dielectrics or/and substrate for graphene) has been described. Dielectric layers fabricated from fluorinated graphene or in combination with graphene oxide are the most promising graphene-based flexible and transparent electronics.