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Highly conductive, monolayer and large-area reduced graphene oxide films fabricated by electrical connection at the two-dimensional boundaries between the tiled graphene oxide flakes
Thin Solid Films 615 (2016) 247–255
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Highly conductive, monolayer and large-area reduced graphene oxide films fabricated by electrical connection at the two-dimensional boundaries between the tiled graphene oxide flakes M. Zikri B. Dzukarnain a, Toshiyuki Takami a, Hibiki Imai a, Toshio Ogino a,b,⁎ a b
Department of Physics, Electrical and Computer Engineering, Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan CREST/JST(Japan Science and Technology Agency), Gobancho-7, Chiyoda-ku, Tokyo 102-0076, Japan
a r t i c l e
i n f o
Article history: Received 26 November 2015 Received in revised form 6 July 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: Graphene oxide Reduced graphene oxide Thermal annealing Catalyst-free graphene synthesis Electrical conductivity Large-area, continuous thin film
a b s t r a c t The demand to fabricate graphene film in large wafer-scale with minimum cost is crucial for device applications, and one of the promising techniques is the reduction of graphene oxide (GO) films. We have investigated morphological changes of large-area, monolayer GO films during methane (CH4)-assisted thermal annealing process and the role of CH4 in the restoration of their electrical conductivity. We have discovered that long reduction process in high CH4 flow rates can contribute to the partial reparation of lattice defects and formation of new multilayer graphene that are stacked at the boundary areas between the reduced GO (rGO) flakes. These multilayer graphene layers with graphitic domains operate as the electrical connection between the separated rGO flakes and create new conduction pathways across the entire films. This phenomenon is more important than the sole reduction process (elimination of oxygen functional groups) in the restoration of electrical conductivity of the continuous rGO films. We have achieved in the fabrication of highly conductive rGO films with a minimum sheet resistance value of 1.01 kΩ/sq. This study exhibits a promising reduction method for the mass production of large-area, continuous rGO films for thin film device applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since graphene, which is a two-dimensional (2D) material made up of only single layer of carbon atoms, was first discovered [1–3], it has attracted worldwide attention due to its remarkable electrical [4,5], mechanical [4,6] and optical properties [4,7] and is expected for various kinds of device applications in the near future. Above all, graphene exhibits potential applications in the area of flexible, transparent electrode and electrically conductive thin-film devices [8–11] owing to its unique one-atom-thick properties. However, the demand to fabricate graphene films in large-size for mass production and to find the most cost-effective fabrication method is still a challenge. Among all graphene fabrication methods [12–17], reduction of graphene oxide (GO) is expected to be a new potential solution for these obstacles. GO can be described as single layer of graphite decorated with random distribution of oxygen functional groups, mostly of epoxy (C\\O\\C) and hydroxyl groups (C\\OH) at the edges and basal planes, combined with non-oxidized regions where sp2 hybridization are preserved [18,19]. These functional groups are the result of oxidizing graphite in a strong acid solution [20,21], where monolayer flakes/ ⁎ Corresponding author. E-mail addresses: [email protected] (M.Z.B. Dzukarnain), [email protected] (T. Ogino).
http://dx.doi.org/10.1016/j.tsf.2016.07.029 0040-6090/© 2016 Elsevier B.V. All rights reserved.
sheets of GO with less than 1 nm of thickness [22] can be obtained in a form of aqueous solution. Hence, we will be able to produce largesize GO films by simple methods, such as spin-coating [23] and dropcasting [23] on desired substrates. This can be the fundamental step to fabricate GO films for further applications in mass production scale. To obtain substrates suitable for the GO film formation, our group proposed a method of changing a negatively-charged Si oxide substrate surface to a positively-charged surface by depositing (3aminopropyl)triethoxylsilane (APTES) self-assembled monolayers (SAMs) in an aqueous solution [24]. As a result, the repulsive force between negatively-charged GO flakes and oxide substrates can be overcome, resulting in a strong attractive force to fabricate large-area monolayer GO films that cover the whole substrate surface. However, GO is almost insulating [23,25] due to the fact that a considerable part of sp2 bonded carbon atoms with non-localized π electrons are lost and sp3 hybridized states are formed during the oxidizing process of graphite, degrading its electrical properties. Therefore, for further applications involving electrical conductivity, GO films need to be converted into reduced graphene oxide (rGO) films. To date, there are various techniques to reduce GO, such as chemical reduction method [26–28] and thermal reduction (annealing) method [29–34] where its electrical conductivity can be restored. Yet, even after reduction process, it is still a challenge to obtain rGO that has near-pristine graphene quality because disruption of the carbon plane occurs during the reduction process to
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remove the oxygen functional groups [22]. Therefore, it is crucial to optimize the reduction process of GO films where high conductivity of rGO films with improved graphitic structure and quality can be obtained for further applications. Thermal annealing method is an effective technique to reduce GO films, especially in large wafer-scale for mass production. Several literature reports show promise in the low temperature thermal annealing method [35–37], where GO films were annealed at temperatures lower than 300 °C [35–37]. However, the demands of a very long reduction time [35], small-area [36] and low electrical conductivity of the conventional rGO films [35,38] are some of the drawbacks that need further improvement. Although Zhao et al. demonstrated synthesis of large-area GO sheets with maximum area that can reach up to several mm2 [39], it is still difficult to control the size and fabricate continuous films due to the randomness of lateral shapes and sizes of each GO flakes [39,40]. Moreover, high densities of voids or vacant areas still exist [24] between the GO flakes that will cause nonuniformity and low electrical conductivity of the films. In our previous work, we have investigated the morphology of GO films annealed in various atmospheres such as methane (CH4) and hydrogen (H2) [24]. Under CH4 atmosphere, electrical conductivity of the rGO films was greatly improved when the voids between GO flakes were filled with carbon-related materials. However, unlike normal chemical vapor deposition (CVD) synthesis of graphene, where transition metals act as the catalyst for the graphene synthesis [12,13,16,17,41], there is still little knowledge on the best condition for the thermal annealing of GO films and the effect of the new graphene synthesis at the edges towards highly conductive and continuous rGO film fabrication. In this study, by focusing on the amount of CH4 flow rate and reduction time, we have investigated the morphological changes of large-area monolayer GO films during CH4-assisted thermal annealing process and the role of CH4 in the restoration of electrical conductivity by comparing it with different reduction processes using another atmosphere. For a highly conductive film, we propose the importance of fabrication of continuous rGO film with improved graphitic crystalline structure by connecting the randomly separated rGO flakes. As a result, we have restored the entire film conductivity and fabricated very conductive rGO films to only about one order of magnitude higher than that of pristine graphene (400 Ω) [1].
GO films were annealed by using atmospheric pressure CVD (AP-CVD, First Nano) equipment. First, the samples were heated in the quartz-tube furnace for 15 min until the temperature reached 1000 °C in Ar flow. Next, GO films were annealed at 1000 °C in CH4 atmosphere. During the reduction process, H2 was used as the catalyst for the dissociation of CH4 gas [43] and for the elimination of oxygen-functional groups [25] while Ar was used as carrier gas. In ‘Experiment1’, GO films were annealed for 30 min in different CH4 flow rate. In ‘Experiment2’, GO films were annealed in different reduction time while keeping the CH4 flow rate constant at 1000 sccm. In ‘Experiment3’, GO films were annealed in only Ar atmosphere, which was similar to as-reported thermal reduction of GO films [44]. H2 was not used in ‘Experiment3’ in order to avoid the etching of rGO flakes from the edges [24,45,46]. However, we used H2 [24,25] in ‘Experiment4’ to investigate the relationship between the film quality and its entire conductivity by effectively eliminating the oxygen functional groups. The details about all reduction processes are summarized in Table 1. After the annealing processes were completed, samples were cooled to room temperature in Ar flow. The morphological changes of the samples were analyzed by using contact mode of atomic force microscopy (AFM) manufactured by Hitachi High-Tech Science (SPI4000/E-sweep). For AFM observation, besides the usual topographic images, frictional images were taken in order to differentiate between the SiO2 surfaces and the rGO flakes after reduction owing to the difference in their hydrophilic or hydrophobic properties. In addition, Nano/Pico Current AFM mode was also used to analyze the local conductivity of the rGO films. Raman spectroscopy of the GO and rGO films were obtained by using a JASCO Laser Raman Spectrophotometer (NRS-300), which is equipped with a 532 nm wavelength laser light, 1800 mm− 1 gratings and a 100 × objective. We investigated the electrical properties of the GO and rGO films using four-point probe transport measurement at room temperature. A total of 10 different measurements were taken for each samples and the average sheet resistance values were calculated. X-ray photoelectron spectroscopy (XPS, Quantera SXM) was used to determine the oxygen compositions of the samples before and after the reduction processes. Finally, the surface of rGO films was observed by using scanning electron microscopy (S-4700, Hitachi High-Tech Science) at room temperature.
2. Experimental procedures
3. Results and discussions
GO water dispersion that has a concentration of about 2.78 wt% was prepared from commercially available high-purity natural graphite powder (99.97%, SEC Carbon, SNO-3000) of 3 mm size based on the modified Hummers' method [20,42]. Si substrates with average size of 5 mm × 5 mm and 300 nm of SiO2 layer were cleaned in a mixed solution of H2SO4 and H2O2 (volume ratio, 3:1) for 10 min at 90 °C, and then ultra-sonicated in deionized (DI) water for 5 min. The SiO2/ Si surfaces were modified into positively-charged surface by liquid phase deposition of a mixture of dehydrated toluene (99.5%) and APTES (98%, SIGMA-ALDRICH) solution [24]. GO films were fabricated by drop-casting 20 μl of GO water dispersion onto the APTES/SiO2/Si substrates. Then, the samples were dried in vacuum for 30 min. In order to remove extra multilayer GO flakes, the samples were ultrasonicated in DI water for 60 s. Finally, only monolayer GO films were prepared [24].
Large-area GO films that cover the whole substrate surface were prepared on the APTES/SiO2/Si substrates [24], as shown in Fig. 1. For each of the GO films, only monolayer GO flakes were observed in AFM images, corresponding to 0.7–1.0 nm of thickness [22,24]. Owing to the oxygen functional groups that were still bonded to both sides of the graphene plane, the apparent thickness before reduction was larger than that of graphene derived from graphite. Fig. 2 shows AFM images of rGO films annealed for 30 min in 4 sccm (a–b), 30 sccm, (c–d) 200 sccm (e–f), 500 sccm (g–h) and 1000 sccm (i–j) of CH4 flow rate at 1000 °C. The top row of Fig. 2 (a, c, e, g, i) represents typical AFM topographic images while the bottom row (b, d, f, h, j) represents the frictional images. For the frictional images, brighter regions correspond to the oxide substrates where large frictional force was observed due to the hydrophilic nature of SiO2 surface. First, after reduction in 4 sccm of CH4 flow rate (Fig. 2a–b), we
Table 1 Details of the annealing parameters for each experiment. Experiment
Annealing temperature [°C]
CH4 [sccm]
H2 [sccm]
Ar [sccm]
Time [min]
1 2 3 4
1000 1000 1000 1000
4,30,100,200,500,1000 1000 0 0
15 15 0 500
500 500 500 500
30 5,10,30,45,60,90 5,10,30,45,60,90 60
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Fig. 1. AFM images of a GO film fabricated on APTES/SiO2/Si surface. (a) Topographic image. (b) Frictional image. (c) Cross section image of the same sample.
can still observe voids or vacant areas at the boundaries between the rGO flakes. The voids can be confirmed from AFM frictional image (Fig. 2b), where the APTES molecules should have been decomposed after being annealed at 1000 °C. However, after increasing the amount of CH4 flow rate to 30 sccm (Fig. 2c), in spite of non-existence of the catalyst, new graphene growth was observed at the edges of rGO flakes, and the gaps between the rGO flakes were filled with continuous rGO films. When CH4 flow was increased gradually, as shown in Fig. 2e onwards, new graphene layers were synthesized and stacked in vertical direction from the edge of the rGO flakes. The new graphene layers continued to grow and its thickness constantly increased in the proportion of CH4 flow rate, resulting in multilayer graphene formation only at the boundary areas. Based on the AFM topographic images, the thickness observed was 2–3 nm (Fig. 2e) at 200 sccm, 5–6 nm (Fig. 2g) at 500 sccm and 7–8 nm (Fig. 2i) at 1000 sccm, respectively. In contrast,
the original monolayer thickness of the rGO flakes remained constant where no graphene growth was observed. Based on Fig. 2j, the multilayer graphene synthesized at the boundaries displays lower frictional force value, which corresponds to higher hydrophobicity than that of the original rGO flakes. We propose that properties of the carbon layers at the boundaries are close to CVD-grown graphene rather than rGO. Furthermore, the difference between the frictional force values is due to the residual oxygen-functional groups that are still bonded at the original rGO flake edges even after the reduction process. We propose that a very low CH4 flow rate is not sufficient for the synthesis of graphene growth from the edges of the rGO flakes. For CVD graphene synthesis on metal surfaces, smaller amount of hydrocarbon will decrease the adsorption of carbon atoms on the metal surface. Therefore, nucleation of graphene can be suppressed, thus improving quality and uniformity of the graphene films [47–49]. However, when
Fig. 2. AFM images of the rGO films annealed for 30 min in different CH4 flow rates. (a, c, e, g, i) AFM topographic images of the rGO films. (b, d, f, h, j) AFM frictional images. (a–b) rGO film annealed in 4 sccm of CH4 flow rate, (c–d) 30 sccm, (e–f) 200 sccm, (g–h) 500 sccm, (i–j) 1000 sccm, respectively.
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the GO films are annealed in a very low CH4 flow rate (4 sccm), there is no catalyst for the deposition of carbon atoms at the GO edges during reduction process. As a result, no graphene synthesis was observed. Raman spectra of the rGO films annealed for 30 min based on CH4 flow rate are shown in Fig. 3a. In typical Raman spectra of the as-deposited GO film, we can observe broad and high D band at 1350 cm−1, which is derived from the lattice disorder or defects [50], as the results of the attachment of oxygen-functional groups and introduction of lattice defects during the oxidation process [51]. However, even after reduction, all Raman spectra display a prominent D band, suggesting that lattice defects still remained. Intensity of G band at 1580 cm− 1 and 2D band at 2700 cm−1, which are derived from graphitic configuration, display a slight increase when the amount of CH4 flow increased. We propose that new graphene growth and partial reparation of the carbon network structure occurred. However, in Fig. 3b, after the CH4 flow rate reach 200 sccm, the intensity ratio of 2D/G bands (I2D/IG) remains constant, showing that there is a limit in the restoration of graphene lattice when all samples were annealed in the same reduction time. Table 2 shows the measured average sheet resistance (ρs) values of the as-deposited GO film and rGO films. When the amount of CH4 flow increased in the 30 min reduction, rGO films exhibited a decrease in the ρs values compared to the as-deposited GO (~ 109 Ω/sq) film. Moreover, as supported by the AFM images, once the continuous rGO films were formed (Fig. 2c–d), the electrical conductivity of rGO films were gradually restored. Although the removal of oxygen-functional groups leads to the improvement of rGO conductivity via thermal annealing [23,52] at high temperature, we propose that the new graphene growth from the edges of rGO flakes plays a greater role in the restoration of rGO conductivity. By maintaining CH4 flow rate of 1000 sccm, GO films were annealed for different reduction time. Figs. 4 and 5 show the AFM images of the rGO films after reduction. Fig. 4a–b shows rGO films annealed for only 5 min and we can observe voids that correlate to the SiO2/Si surfaces. A very short annealing time is not sufficient for the new graphene synthesis because there was no catalyst involved to enhance the graphene nucleation on the surface of the GO flakes. However, by using a very high volume of CH4 flow rate during the annealing process for 10 min of reduction time, we can observe new graphene growth from the rGO edges, resulting in the formation of uniform and continuous rGO films without any voids between the rGO flakes (Fig. 4d). Similar to the previous experiment using 30 min annealing, new graphene layers were synthesized and stacked in the vertical direction around the rGO edges after increasing the reduction time, leading to the formation of
Table 2 Average ρs values of the as-deposited GO and rGO films annealed at different CH4 flow rates for 30 min. Sample
CH4 flow-rate [sccm]
Sheet resistance, ρs [kΩ/sq]
GO rGO 4 sccm rGO 30 sccm rGO 100 sccm rGO 200 sccm rGO 500 sccm rGO 1000 sccm
0 4 30 100 200 500 1000
5.80 × 106 38.7 14.6 8.22 6.97 4.77 4.18
multilayer graphene in the boundary areas, as shown in Fig. 4e–f. Based on the AFM topographic images of Fig. 4e–f and cross section images in Fig. 5, the stacking multilayer graphene thickness at the gap areas was 1–2 nm for 10 min of reduction time (Fig. 5a), ~ 14 nm for 60 min (Fig. 5b), and ~20 nm (Fig. 5c) for 90 min, respectively. Nonetheless, the thickness of the original rGO monolayer flakes remained unaffected even after a very long annealing process. We propose that difference in the natures between the original GO, which was derived from graphite, and the new graphene grown at the rGO edges, which was similar to CVD-synthesized graphene, affected the difference in their morphological changes after reduction process. Furthermore, the difference between their hydrophobic properties displayed in the AFM frictional images after reduction supports our assumption. Next, Fig. 6a shows comparison of Raman spectra between the asdeposited GO film and rGO films annealed for different reduction time, while Fig. 6b shows the details of the 2D band intensities. Intensity enhancement and decrease in the widths of G and 2D bands of Fig. 6a–b suggest that there might be improvement to the graphitic crystalline structure through the restoration of rGO interatomic distance [53] after long reduction process in CH4 atmosphere. Next, Fig. 6c shows that the D/G band intensity ratio (ID/IG) decreased with the increment of reduction time. ID/IG provides a measure of the disorder degree and the ratio of the graphitic crystalline areas as expressed by sp3/sp2 carbon atom ratio by using the Tuinstra-Koening relation [50,54]. The decrease in ID/IG means that longer reduction time leads to increase in new graphene synthesis from the edge of rGO where more graphene-like domains formed. However, almost no change in the D band intensities was observed where every spectra still exhibit its strong intensity. Reduction in CH4 atmosphere may introduce new sp2 carbon atoms and restore the defects in the rGO structures, but it is still difficult to reconstruct defective conjugated network to pristine graphitic one. Besides, the disruption of graphene lattice might as well occur [55],
Fig. 3. (a) Raman spectra comparison of the rGO films annealed for 30 min at different CH4 flow rates. (b) Intensity ratio of 2D/G bands (I2D/IG) as a function of CH4 flow rate. Inset displays a Raman spectrum of rGO film annealed in 1000 sccm of CH4 flow rate.
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Fig. 4. AFM images of the as-deposited GO film and rGO films annealed in CH4 atmosphere for different reduction times while maintaining the same CH4 flow rate (1000 sccm). Topographic image (a) and frictional image (b) of the rGO film annealed for 5 min. (c–f) 3D morphological changes of GO film annealed for different reduction times. (c) 3D AFM image of GO film before annealing, (d) rGO film annealed for 10 min, (e) 60 min, (f) 90 min, respectively.
owing to very long reduction time at high temperatures. This is because the energy needed for the removal of C\\O bonds is higher than the energy to break C\\C bonds in graphene lattice [56]. Furthermore, other functional groups such as carbonyl (C_O) ones at the edges of graphene plane are more stable and are difficult to remove even after thermal annealing around 1000 °C [57].
We investigated degree of reduction of GO films annealed at 1000 °C for different reduction time by measuring C 1s XPS spectra, as shown in Fig. 7. When the GO films were annealed for only 10 min, almost all oxygen-related peaks such as C\\O (hydroxyl; 286.9 eV) bonds, C\\O\\C (epoxy; 288.1 eV) bonds and O_C\\O (carboxyl; 289.0 eV) bonds, were not observed, suggesting that high temperature annealing
Fig. 5. AFM cross section images show the thickness of stacked multilayer graphene at the boundaries between the tiled rGO flakes of rGO films annealed for (a) 10 min, (b) 60 min and (c) 90 min, respectively.
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Fig. 6. (a) Comparison of Raman spectra between as-deposited GO and rGO films annealed for different reduction times. (b) Comparison of 2D bands for different reduction times. (c) Intensity ratio of D/G bands as a function of the reduction time. Insets of (b) and (c) show a Raman spectrum of the rGO film annealed for 90 min. Average values of D and G band intensities were calculated from 6 different measurements and the black lines show the range of the calculated values.
was effective for the removal of oxygen-functional groups [23,31]. Because the surfaces of the rGO areas are much lower and their roughness is also smaller than the boundary areas, as shown in Fig. 5, the newly-synthesized graphene layers were deposited almost only in the boundaries areas, not on the rGO flakes. Therefore, the XPS results,
which are surface sensitive, indicate that oxygen species on the original GO films were eliminated upon the high temperature CH4 annealing. Table 3 shows average ρs values of the as-deposited GO film and rGO films annealed for different reduction time. Based on the ρs values, electrical conductivity of rGO films was remarkably restored by reduction time of 10 min. Moreover, the ρs values continued to decrease when the reduction time increased and more graphene layers were stacked at the boundaries. The lowest ρs value achieved was 1.01 kΩ/sq for 90 min of reduction time, which was 6 orders of magnitude better than the ρs value of the original GO film. As supported by the AFM images of the rGO film treated for 10 min of reduction time (Fig. 4d), we can conclude that once continuous rGO films were fabricated in the boundary area, conductivity of rGO films is greatly improved. The conduction mechanism in graphene is generally related to its structural geometry that includes lattice defects and bonded functional groups. In the case of GO sheet before the reduction process, preserved small conductive sp2 domains are isolated by large, dominant insulating sp3 domains in the individual GO plane [25,58] of each flake. The finite sp2 cluster areas and the low carrier density of the insulating GO sheet Table 3 Average ρs values of the as-deposited GO and rGO films annealed for different reduction times in CH4 atmosphere.
Fig. 7. C 1s XPS spectra of the as-deposited GO film and rGO films annealed in 1000 sccm of CH4 flow for 10 and 30 min. Peaks with binding energies of 284.8 eV, 286.9 eV, 288.1 eV and 289.0 eV correspond to C\ \C bonds, C\ \O bonds, C\ \O\ \C bonds and O_C\ \O bonds, respectively.
Sample
Reduction time [min]
Sheet resistance, ρs [kΩ/sq]
GO rGO-CH4 rGO-CH4 rGO-CH4 rGO-CH4 rGO-CH4 rGO-CH4
0 5 10 30 45 60 90
5.80 × 106 5.05 × 102 6.99 4.40 4.18 3.07 1.01
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Fig. 8. (a) Comparison of average ID/IG ratio values with the ρs values. The black lines show the range of the measured ID/IG ratio values. (b) SEM image of a rGO film annealed for 60 min at 1000 sccm of CH4 flow rate, which demonstrates the formation of the electrical connection between the rGO flakes. Inset shows an AFM topographic image of the same sample. (c) AFM topographic image of a rGO film annealed by a condition similar to (b). (e) AFM current image of (c). (d) and (f) magnified images of the samples in panels (c) and (e), respectively.
compared to pristine graphene also influence its low conductivity [32, 59]. Moreover, the existence of edge defects and residual oxygen functional groups lower the total conductivity [60]. After the reduction process, due to the elimination of oxygen-functional groups and new sp2 carbon atoms that are being introduced, the size of sp2 carbon clusters increases. Therefore, by connecting the graphene lattice domains, new electrical pathways among the sp2 domains form. Fig. 8a shows the comparison of the ID/IG ratio with the average ρs
values. We can observe that ρs value dropped along with the ID/IG ratio, suggesting that the increase in rGO sp2 cluster size caused by the new graphene synthesis and restoration of sp2 carbon lattice contribute to the efficient restoration of rGO conductivity [61]. Furthermore, in this study, we are able to synthesize thicker multilayer graphene at the boundary areas between the rGO flakes when increasing the reduction time of GO films in CH4 atmosphere. Although the quality of these graphene layers might not be very close to the mechanically exfoliated
Fig. 9. (a) Comparison of Raman spectra between the as-deposited GO and rGO films annealed in Ar atmosphere for different reduction times. Inset of (a) shows an AFM frictional image of the rGO film reduced in Ar atmosphere for 60 min. (b) Comparison of Raman spectra between the rGO film annealed in 3 different atmospheres. All rGO films were annealed for 60 min.
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Table 4 Average ρs values of the as-deposited GO and rGO films annealed for different reduction times in Ar atmosphere. Sample
Reduction time [min]
Sheet resistance, ρs [kΩ/sq]
GO rGO-Ar rGO-Ar rGO-Ar rGO-Ar rGO-Ar rGO-Ar
0 5 10 30 45 60 90
5.80 × 106 2.90 × 103 4.31 × 103 6.89 × 102 2.09 × 102 1.12 × 103 4.27 × 103
or metal-assisted CVD-grown graphene owing to their catalyst-free synthesis, they partially preserve their graphitic domains because the ρs values are decreased after these layers were grown. As a result, these multilayer graphene operates as the new electrical connection between the originally separated rGO flakes, as shown in the SEM image of Fig. 8b. Therefore, the high resistivity at the flake edges can be overcome, resulting in the high conductivity across the entire rGO films. Furthermore, Fig. 8c–f shows the conductive AFM images of the rGO film annealed by the condition similar to Fig. 8b. We can observe uniform distribution of current across the whole film, regardless of the difference between the original rGO flakes and the new graphene layers. However, we can still confirm the existence of hole-type defects of 100– 500 nm size in the rGO films that appear as the insulating regions in the current images. These large defects might be the result of the unrepairable dense defects that were introduced during the early stages of the oxidation process. Interestingly, the conductive properties of our continuous rGO films formed by the electrical connection are not affected by this type of defects. To confirm our assumptions in ‘Experiment 1’ and ‘Experiment 2’, we conducted thermal reduction experiment of GO films only in Ar or H2 [24,25] atmosphere. Fig. 8a shows comparison of Raman spectra of the rGO films annealed in Ar atmosphere for different reduction time. We can observe that, regardless of reduction time, there was almost no change in the intensity of G and 2D bands. These results suggest that, without CH4 flow, there was no restoration of carbon lattice defects on the graphene plane and no synthesis of graphene at the rGO edges, as confirmed by the AFM frictional images in the inset of Fig. 9a. To clarify the CH4 role, we plotted Raman spectra of the rGO films annealed in 3 different types of atmosphere for 60 min, as shown in Fig. 9b. There was a huge difference between the intensities of G and 2D bands of the rGO films, where the rGO films annealed in CH4 atmosphere exhibit much higher G and 2D band intensities. These results are in agreement with our assumption that rGO graphitic crystalline structure can be partially restored and increase sp2 cluster size by the annealing in CH4 atmosphere. Furthermore, ρs values of the rGO films annealed in H2 and Ar exhibit low electrical conductivity (~103 kΩ/sq) compared with the rGO films annealed in CH4 atmosphere, although they were annealed for the same reduction time, as shown in Table 4 and Table 5. For 60 min of reduction time, ρs value of the rGO films annealed in only Ar or H2 atmosphere is 3 orders of magnitudes higher than the ρs value of rGO films annealed in CH4 atmosphere. In the Raman spectra of Fig. 9b, we can locally fabricate less defective rGO films via the H2-assisted annealing by effectively removing the oxygen-functional groups, which leads to the healing of atomic level
Table 5 Comparison of the average ρs values of the as-deposited GO and rGO films annealed in CH4, H2 or Ar atmosphere for 60 min. Sample
Reduction time [min]
Sheet resistance, ρs [kΩ/sq]
GO rGO-CH4 rGO-Ar rGO-H2
0 60 60 60
5.80 × 106 3.07 1.12 × 103 2.54 × 103
defects [25]. However, the entire conductivity of the H2-annealed film is low compared with the rGO films annealed in CH4 atmosphere. We suggest that it is not sufficient to completely restore entire electrical conductivity of the rGO films without connecting the individual rGO flakes by forming continuous films. Furthermore, etching of the rGO flakes by H2 [24,45,46] also increases the resistivity at the graphene edges and affects the entire conductivity. 4. Conclusions In conclusion, towards the fabrication of large-area continuous rGO films, large amount of CH4 flow is needed for the new graphene synthesis at the GO edges due to the non-existence of catalyst. We realized that this phenomenon contradicted with the common CVD synthesis of graphene on transition metals. Besides, it is suggested that the original GO/rGO edges act as the starting points for the new graphene synthesis and we found that graphene continue to grow and stack in vertical direction from the GO/rGO edges, resulting in the formation of multilayer graphene only in the boundary areas between the flakes. Raman spectrum results demonstrate that the increase in the reduction time in CH4 atmosphere and the synthesis of multilayer graphene helped to partially restore the lattice defects and increase the size of graphene-like domains in rGO crystalline structure. However, there is still a challenge to completely heal the lattice defects and produce rGO films comparable to graphene derived from graphite, where the disruption of carbon-network might occur during the removal of functional groups from the GO plane owing to the high annealing temperature and long reduction time. Sheet resistance values show that restoration of electrical conductivity of rGO films is attributed to the formation of continuous rGO films. It has also been confirmed that the restoration of electrical conductivity is proportional to the amount of multilayer graphene synthesized and not solely from the reduction process, where this phenomenon has been confirmed by the thermal annealing in various atmospheres. We can certify that the formation of new conduction pathways between the separated rGO flakes through multilayer graphene formation and the restoration of lattice defects during the reduction process play important roles in the formation of highly conductive films. In spite of the existence of hole defects at the basal plane, the overall conductivity of the continuous rGO films are unaffected. Finally, this study displays a potential reduction method for mass production of large-area and continuous rGO films towards thin-film device applications. Author contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by Grant-in-Aid for scientific research (15K13361) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Authors thank Yokohama National University Instrumental Analysis Center (IA Center) for providing the XPS instrument. Authors also thank Daichi Yamaura and Ryosuke Kimura regarding the SEM, AFM measurements and useful discussions. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Highly conductive, monolayer and large-area reduced graphene oxide films fabricated by electrical connection at the two-dimensional boundaries between the tiled graphene oxide flakes M. Zikri B. Dzukarnain a, Toshiyuki Takami a, Hibiki Imai a, Toshio Ogino a,b,⁎ a b
Department of Physics, Electrical and Computer Engineering, Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan CREST/JST(Japan Science and Technology Agency), Gobancho-7, Chiyoda-ku, Tokyo 102-0076, Japan
a r t i c l e
i n f o
Article history: Received 26 November 2015 Received in revised form 6 July 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: Graphene oxide Reduced graphene oxide Thermal annealing Catalyst-free graphene synthesis Electrical conductivity Large-area, continuous thin film
a b s t r a c t The demand to fabricate graphene film in large wafer-scale with minimum cost is crucial for device applications, and one of the promising techniques is the reduction of graphene oxide (GO) films. We have investigated morphological changes of large-area, monolayer GO films during methane (CH4)-assisted thermal annealing process and the role of CH4 in the restoration of their electrical conductivity. We have discovered that long reduction process in high CH4 flow rates can contribute to the partial reparation of lattice defects and formation of new multilayer graphene that are stacked at the boundary areas between the reduced GO (rGO) flakes. These multilayer graphene layers with graphitic domains operate as the electrical connection between the separated rGO flakes and create new conduction pathways across the entire films. This phenomenon is more important than the sole reduction process (elimination of oxygen functional groups) in the restoration of electrical conductivity of the continuous rGO films. We have achieved in the fabrication of highly conductive rGO films with a minimum sheet resistance value of 1.01 kΩ/sq. This study exhibits a promising reduction method for the mass production of large-area, continuous rGO films for thin film device applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since graphene, which is a two-dimensional (2D) material made up of only single layer of carbon atoms, was first discovered [1–3], it has attracted worldwide attention due to its remarkable electrical [4,5], mechanical [4,6] and optical properties [4,7] and is expected for various kinds of device applications in the near future. Above all, graphene exhibits potential applications in the area of flexible, transparent electrode and electrically conductive thin-film devices [8–11] owing to its unique one-atom-thick properties. However, the demand to fabricate graphene films in large-size for mass production and to find the most cost-effective fabrication method is still a challenge. Among all graphene fabrication methods [12–17], reduction of graphene oxide (GO) is expected to be a new potential solution for these obstacles. GO can be described as single layer of graphite decorated with random distribution of oxygen functional groups, mostly of epoxy (C\\O\\C) and hydroxyl groups (C\\OH) at the edges and basal planes, combined with non-oxidized regions where sp2 hybridization are preserved [18,19]. These functional groups are the result of oxidizing graphite in a strong acid solution [20,21], where monolayer flakes/ ⁎ Corresponding author. E-mail addresses: [email protected] (M.Z.B. Dzukarnain), [email protected] (T. Ogino).
http://dx.doi.org/10.1016/j.tsf.2016.07.029 0040-6090/© 2016 Elsevier B.V. All rights reserved.
sheets of GO with less than 1 nm of thickness [22] can be obtained in a form of aqueous solution. Hence, we will be able to produce largesize GO films by simple methods, such as spin-coating [23] and dropcasting [23] on desired substrates. This can be the fundamental step to fabricate GO films for further applications in mass production scale. To obtain substrates suitable for the GO film formation, our group proposed a method of changing a negatively-charged Si oxide substrate surface to a positively-charged surface by depositing (3aminopropyl)triethoxylsilane (APTES) self-assembled monolayers (SAMs) in an aqueous solution [24]. As a result, the repulsive force between negatively-charged GO flakes and oxide substrates can be overcome, resulting in a strong attractive force to fabricate large-area monolayer GO films that cover the whole substrate surface. However, GO is almost insulating [23,25] due to the fact that a considerable part of sp2 bonded carbon atoms with non-localized π electrons are lost and sp3 hybridized states are formed during the oxidizing process of graphite, degrading its electrical properties. Therefore, for further applications involving electrical conductivity, GO films need to be converted into reduced graphene oxide (rGO) films. To date, there are various techniques to reduce GO, such as chemical reduction method [26–28] and thermal reduction (annealing) method [29–34] where its electrical conductivity can be restored. Yet, even after reduction process, it is still a challenge to obtain rGO that has near-pristine graphene quality because disruption of the carbon plane occurs during the reduction process to
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remove the oxygen functional groups [22]. Therefore, it is crucial to optimize the reduction process of GO films where high conductivity of rGO films with improved graphitic structure and quality can be obtained for further applications. Thermal annealing method is an effective technique to reduce GO films, especially in large wafer-scale for mass production. Several literature reports show promise in the low temperature thermal annealing method [35–37], where GO films were annealed at temperatures lower than 300 °C [35–37]. However, the demands of a very long reduction time [35], small-area [36] and low electrical conductivity of the conventional rGO films [35,38] are some of the drawbacks that need further improvement. Although Zhao et al. demonstrated synthesis of large-area GO sheets with maximum area that can reach up to several mm2 [39], it is still difficult to control the size and fabricate continuous films due to the randomness of lateral shapes and sizes of each GO flakes [39,40]. Moreover, high densities of voids or vacant areas still exist [24] between the GO flakes that will cause nonuniformity and low electrical conductivity of the films. In our previous work, we have investigated the morphology of GO films annealed in various atmospheres such as methane (CH4) and hydrogen (H2) [24]. Under CH4 atmosphere, electrical conductivity of the rGO films was greatly improved when the voids between GO flakes were filled with carbon-related materials. However, unlike normal chemical vapor deposition (CVD) synthesis of graphene, where transition metals act as the catalyst for the graphene synthesis [12,13,16,17,41], there is still little knowledge on the best condition for the thermal annealing of GO films and the effect of the new graphene synthesis at the edges towards highly conductive and continuous rGO film fabrication. In this study, by focusing on the amount of CH4 flow rate and reduction time, we have investigated the morphological changes of large-area monolayer GO films during CH4-assisted thermal annealing process and the role of CH4 in the restoration of electrical conductivity by comparing it with different reduction processes using another atmosphere. For a highly conductive film, we propose the importance of fabrication of continuous rGO film with improved graphitic crystalline structure by connecting the randomly separated rGO flakes. As a result, we have restored the entire film conductivity and fabricated very conductive rGO films to only about one order of magnitude higher than that of pristine graphene (400 Ω) [1].
GO films were annealed by using atmospheric pressure CVD (AP-CVD, First Nano) equipment. First, the samples were heated in the quartz-tube furnace for 15 min until the temperature reached 1000 °C in Ar flow. Next, GO films were annealed at 1000 °C in CH4 atmosphere. During the reduction process, H2 was used as the catalyst for the dissociation of CH4 gas [43] and for the elimination of oxygen-functional groups [25] while Ar was used as carrier gas. In ‘Experiment1’, GO films were annealed for 30 min in different CH4 flow rate. In ‘Experiment2’, GO films were annealed in different reduction time while keeping the CH4 flow rate constant at 1000 sccm. In ‘Experiment3’, GO films were annealed in only Ar atmosphere, which was similar to as-reported thermal reduction of GO films [44]. H2 was not used in ‘Experiment3’ in order to avoid the etching of rGO flakes from the edges [24,45,46]. However, we used H2 [24,25] in ‘Experiment4’ to investigate the relationship between the film quality and its entire conductivity by effectively eliminating the oxygen functional groups. The details about all reduction processes are summarized in Table 1. After the annealing processes were completed, samples were cooled to room temperature in Ar flow. The morphological changes of the samples were analyzed by using contact mode of atomic force microscopy (AFM) manufactured by Hitachi High-Tech Science (SPI4000/E-sweep). For AFM observation, besides the usual topographic images, frictional images were taken in order to differentiate between the SiO2 surfaces and the rGO flakes after reduction owing to the difference in their hydrophilic or hydrophobic properties. In addition, Nano/Pico Current AFM mode was also used to analyze the local conductivity of the rGO films. Raman spectroscopy of the GO and rGO films were obtained by using a JASCO Laser Raman Spectrophotometer (NRS-300), which is equipped with a 532 nm wavelength laser light, 1800 mm− 1 gratings and a 100 × objective. We investigated the electrical properties of the GO and rGO films using four-point probe transport measurement at room temperature. A total of 10 different measurements were taken for each samples and the average sheet resistance values were calculated. X-ray photoelectron spectroscopy (XPS, Quantera SXM) was used to determine the oxygen compositions of the samples before and after the reduction processes. Finally, the surface of rGO films was observed by using scanning electron microscopy (S-4700, Hitachi High-Tech Science) at room temperature.
2. Experimental procedures
3. Results and discussions
GO water dispersion that has a concentration of about 2.78 wt% was prepared from commercially available high-purity natural graphite powder (99.97%, SEC Carbon, SNO-3000) of 3 mm size based on the modified Hummers' method [20,42]. Si substrates with average size of 5 mm × 5 mm and 300 nm of SiO2 layer were cleaned in a mixed solution of H2SO4 and H2O2 (volume ratio, 3:1) for 10 min at 90 °C, and then ultra-sonicated in deionized (DI) water for 5 min. The SiO2/ Si surfaces were modified into positively-charged surface by liquid phase deposition of a mixture of dehydrated toluene (99.5%) and APTES (98%, SIGMA-ALDRICH) solution [24]. GO films were fabricated by drop-casting 20 μl of GO water dispersion onto the APTES/SiO2/Si substrates. Then, the samples were dried in vacuum for 30 min. In order to remove extra multilayer GO flakes, the samples were ultrasonicated in DI water for 60 s. Finally, only monolayer GO films were prepared [24].
Large-area GO films that cover the whole substrate surface were prepared on the APTES/SiO2/Si substrates [24], as shown in Fig. 1. For each of the GO films, only monolayer GO flakes were observed in AFM images, corresponding to 0.7–1.0 nm of thickness [22,24]. Owing to the oxygen functional groups that were still bonded to both sides of the graphene plane, the apparent thickness before reduction was larger than that of graphene derived from graphite. Fig. 2 shows AFM images of rGO films annealed for 30 min in 4 sccm (a–b), 30 sccm, (c–d) 200 sccm (e–f), 500 sccm (g–h) and 1000 sccm (i–j) of CH4 flow rate at 1000 °C. The top row of Fig. 2 (a, c, e, g, i) represents typical AFM topographic images while the bottom row (b, d, f, h, j) represents the frictional images. For the frictional images, brighter regions correspond to the oxide substrates where large frictional force was observed due to the hydrophilic nature of SiO2 surface. First, after reduction in 4 sccm of CH4 flow rate (Fig. 2a–b), we
Table 1 Details of the annealing parameters for each experiment. Experiment
Annealing temperature [°C]
CH4 [sccm]
H2 [sccm]
Ar [sccm]
Time [min]
1 2 3 4
1000 1000 1000 1000
4,30,100,200,500,1000 1000 0 0
15 15 0 500
500 500 500 500
30 5,10,30,45,60,90 5,10,30,45,60,90 60
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Fig. 1. AFM images of a GO film fabricated on APTES/SiO2/Si surface. (a) Topographic image. (b) Frictional image. (c) Cross section image of the same sample.
can still observe voids or vacant areas at the boundaries between the rGO flakes. The voids can be confirmed from AFM frictional image (Fig. 2b), where the APTES molecules should have been decomposed after being annealed at 1000 °C. However, after increasing the amount of CH4 flow rate to 30 sccm (Fig. 2c), in spite of non-existence of the catalyst, new graphene growth was observed at the edges of rGO flakes, and the gaps between the rGO flakes were filled with continuous rGO films. When CH4 flow was increased gradually, as shown in Fig. 2e onwards, new graphene layers were synthesized and stacked in vertical direction from the edge of the rGO flakes. The new graphene layers continued to grow and its thickness constantly increased in the proportion of CH4 flow rate, resulting in multilayer graphene formation only at the boundary areas. Based on the AFM topographic images, the thickness observed was 2–3 nm (Fig. 2e) at 200 sccm, 5–6 nm (Fig. 2g) at 500 sccm and 7–8 nm (Fig. 2i) at 1000 sccm, respectively. In contrast,
the original monolayer thickness of the rGO flakes remained constant where no graphene growth was observed. Based on Fig. 2j, the multilayer graphene synthesized at the boundaries displays lower frictional force value, which corresponds to higher hydrophobicity than that of the original rGO flakes. We propose that properties of the carbon layers at the boundaries are close to CVD-grown graphene rather than rGO. Furthermore, the difference between the frictional force values is due to the residual oxygen-functional groups that are still bonded at the original rGO flake edges even after the reduction process. We propose that a very low CH4 flow rate is not sufficient for the synthesis of graphene growth from the edges of the rGO flakes. For CVD graphene synthesis on metal surfaces, smaller amount of hydrocarbon will decrease the adsorption of carbon atoms on the metal surface. Therefore, nucleation of graphene can be suppressed, thus improving quality and uniformity of the graphene films [47–49]. However, when
Fig. 2. AFM images of the rGO films annealed for 30 min in different CH4 flow rates. (a, c, e, g, i) AFM topographic images of the rGO films. (b, d, f, h, j) AFM frictional images. (a–b) rGO film annealed in 4 sccm of CH4 flow rate, (c–d) 30 sccm, (e–f) 200 sccm, (g–h) 500 sccm, (i–j) 1000 sccm, respectively.
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the GO films are annealed in a very low CH4 flow rate (4 sccm), there is no catalyst for the deposition of carbon atoms at the GO edges during reduction process. As a result, no graphene synthesis was observed. Raman spectra of the rGO films annealed for 30 min based on CH4 flow rate are shown in Fig. 3a. In typical Raman spectra of the as-deposited GO film, we can observe broad and high D band at 1350 cm−1, which is derived from the lattice disorder or defects [50], as the results of the attachment of oxygen-functional groups and introduction of lattice defects during the oxidation process [51]. However, even after reduction, all Raman spectra display a prominent D band, suggesting that lattice defects still remained. Intensity of G band at 1580 cm− 1 and 2D band at 2700 cm−1, which are derived from graphitic configuration, display a slight increase when the amount of CH4 flow increased. We propose that new graphene growth and partial reparation of the carbon network structure occurred. However, in Fig. 3b, after the CH4 flow rate reach 200 sccm, the intensity ratio of 2D/G bands (I2D/IG) remains constant, showing that there is a limit in the restoration of graphene lattice when all samples were annealed in the same reduction time. Table 2 shows the measured average sheet resistance (ρs) values of the as-deposited GO film and rGO films. When the amount of CH4 flow increased in the 30 min reduction, rGO films exhibited a decrease in the ρs values compared to the as-deposited GO (~ 109 Ω/sq) film. Moreover, as supported by the AFM images, once the continuous rGO films were formed (Fig. 2c–d), the electrical conductivity of rGO films were gradually restored. Although the removal of oxygen-functional groups leads to the improvement of rGO conductivity via thermal annealing [23,52] at high temperature, we propose that the new graphene growth from the edges of rGO flakes plays a greater role in the restoration of rGO conductivity. By maintaining CH4 flow rate of 1000 sccm, GO films were annealed for different reduction time. Figs. 4 and 5 show the AFM images of the rGO films after reduction. Fig. 4a–b shows rGO films annealed for only 5 min and we can observe voids that correlate to the SiO2/Si surfaces. A very short annealing time is not sufficient for the new graphene synthesis because there was no catalyst involved to enhance the graphene nucleation on the surface of the GO flakes. However, by using a very high volume of CH4 flow rate during the annealing process for 10 min of reduction time, we can observe new graphene growth from the rGO edges, resulting in the formation of uniform and continuous rGO films without any voids between the rGO flakes (Fig. 4d). Similar to the previous experiment using 30 min annealing, new graphene layers were synthesized and stacked in the vertical direction around the rGO edges after increasing the reduction time, leading to the formation of
Table 2 Average ρs values of the as-deposited GO and rGO films annealed at different CH4 flow rates for 30 min. Sample
CH4 flow-rate [sccm]
Sheet resistance, ρs [kΩ/sq]
GO rGO 4 sccm rGO 30 sccm rGO 100 sccm rGO 200 sccm rGO 500 sccm rGO 1000 sccm
0 4 30 100 200 500 1000
5.80 × 106 38.7 14.6 8.22 6.97 4.77 4.18
multilayer graphene in the boundary areas, as shown in Fig. 4e–f. Based on the AFM topographic images of Fig. 4e–f and cross section images in Fig. 5, the stacking multilayer graphene thickness at the gap areas was 1–2 nm for 10 min of reduction time (Fig. 5a), ~ 14 nm for 60 min (Fig. 5b), and ~20 nm (Fig. 5c) for 90 min, respectively. Nonetheless, the thickness of the original rGO monolayer flakes remained unaffected even after a very long annealing process. We propose that difference in the natures between the original GO, which was derived from graphite, and the new graphene grown at the rGO edges, which was similar to CVD-synthesized graphene, affected the difference in their morphological changes after reduction process. Furthermore, the difference between their hydrophobic properties displayed in the AFM frictional images after reduction supports our assumption. Next, Fig. 6a shows comparison of Raman spectra between the asdeposited GO film and rGO films annealed for different reduction time, while Fig. 6b shows the details of the 2D band intensities. Intensity enhancement and decrease in the widths of G and 2D bands of Fig. 6a–b suggest that there might be improvement to the graphitic crystalline structure through the restoration of rGO interatomic distance [53] after long reduction process in CH4 atmosphere. Next, Fig. 6c shows that the D/G band intensity ratio (ID/IG) decreased with the increment of reduction time. ID/IG provides a measure of the disorder degree and the ratio of the graphitic crystalline areas as expressed by sp3/sp2 carbon atom ratio by using the Tuinstra-Koening relation [50,54]. The decrease in ID/IG means that longer reduction time leads to increase in new graphene synthesis from the edge of rGO where more graphene-like domains formed. However, almost no change in the D band intensities was observed where every spectra still exhibit its strong intensity. Reduction in CH4 atmosphere may introduce new sp2 carbon atoms and restore the defects in the rGO structures, but it is still difficult to reconstruct defective conjugated network to pristine graphitic one. Besides, the disruption of graphene lattice might as well occur [55],
Fig. 3. (a) Raman spectra comparison of the rGO films annealed for 30 min at different CH4 flow rates. (b) Intensity ratio of 2D/G bands (I2D/IG) as a function of CH4 flow rate. Inset displays a Raman spectrum of rGO film annealed in 1000 sccm of CH4 flow rate.
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Fig. 4. AFM images of the as-deposited GO film and rGO films annealed in CH4 atmosphere for different reduction times while maintaining the same CH4 flow rate (1000 sccm). Topographic image (a) and frictional image (b) of the rGO film annealed for 5 min. (c–f) 3D morphological changes of GO film annealed for different reduction times. (c) 3D AFM image of GO film before annealing, (d) rGO film annealed for 10 min, (e) 60 min, (f) 90 min, respectively.
owing to very long reduction time at high temperatures. This is because the energy needed for the removal of C\\O bonds is higher than the energy to break C\\C bonds in graphene lattice [56]. Furthermore, other functional groups such as carbonyl (C_O) ones at the edges of graphene plane are more stable and are difficult to remove even after thermal annealing around 1000 °C [57].
We investigated degree of reduction of GO films annealed at 1000 °C for different reduction time by measuring C 1s XPS spectra, as shown in Fig. 7. When the GO films were annealed for only 10 min, almost all oxygen-related peaks such as C\\O (hydroxyl; 286.9 eV) bonds, C\\O\\C (epoxy; 288.1 eV) bonds and O_C\\O (carboxyl; 289.0 eV) bonds, were not observed, suggesting that high temperature annealing
Fig. 5. AFM cross section images show the thickness of stacked multilayer graphene at the boundaries between the tiled rGO flakes of rGO films annealed for (a) 10 min, (b) 60 min and (c) 90 min, respectively.
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Fig. 6. (a) Comparison of Raman spectra between as-deposited GO and rGO films annealed for different reduction times. (b) Comparison of 2D bands for different reduction times. (c) Intensity ratio of D/G bands as a function of the reduction time. Insets of (b) and (c) show a Raman spectrum of the rGO film annealed for 90 min. Average values of D and G band intensities were calculated from 6 different measurements and the black lines show the range of the calculated values.
was effective for the removal of oxygen-functional groups [23,31]. Because the surfaces of the rGO areas are much lower and their roughness is also smaller than the boundary areas, as shown in Fig. 5, the newly-synthesized graphene layers were deposited almost only in the boundaries areas, not on the rGO flakes. Therefore, the XPS results,
which are surface sensitive, indicate that oxygen species on the original GO films were eliminated upon the high temperature CH4 annealing. Table 3 shows average ρs values of the as-deposited GO film and rGO films annealed for different reduction time. Based on the ρs values, electrical conductivity of rGO films was remarkably restored by reduction time of 10 min. Moreover, the ρs values continued to decrease when the reduction time increased and more graphene layers were stacked at the boundaries. The lowest ρs value achieved was 1.01 kΩ/sq for 90 min of reduction time, which was 6 orders of magnitude better than the ρs value of the original GO film. As supported by the AFM images of the rGO film treated for 10 min of reduction time (Fig. 4d), we can conclude that once continuous rGO films were fabricated in the boundary area, conductivity of rGO films is greatly improved. The conduction mechanism in graphene is generally related to its structural geometry that includes lattice defects and bonded functional groups. In the case of GO sheet before the reduction process, preserved small conductive sp2 domains are isolated by large, dominant insulating sp3 domains in the individual GO plane [25,58] of each flake. The finite sp2 cluster areas and the low carrier density of the insulating GO sheet Table 3 Average ρs values of the as-deposited GO and rGO films annealed for different reduction times in CH4 atmosphere.
Fig. 7. C 1s XPS spectra of the as-deposited GO film and rGO films annealed in 1000 sccm of CH4 flow for 10 and 30 min. Peaks with binding energies of 284.8 eV, 286.9 eV, 288.1 eV and 289.0 eV correspond to C\ \C bonds, C\ \O bonds, C\ \O\ \C bonds and O_C\ \O bonds, respectively.
Sample
Reduction time [min]
Sheet resistance, ρs [kΩ/sq]
GO rGO-CH4 rGO-CH4 rGO-CH4 rGO-CH4 rGO-CH4 rGO-CH4
0 5 10 30 45 60 90
5.80 × 106 5.05 × 102 6.99 4.40 4.18 3.07 1.01
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Fig. 8. (a) Comparison of average ID/IG ratio values with the ρs values. The black lines show the range of the measured ID/IG ratio values. (b) SEM image of a rGO film annealed for 60 min at 1000 sccm of CH4 flow rate, which demonstrates the formation of the electrical connection between the rGO flakes. Inset shows an AFM topographic image of the same sample. (c) AFM topographic image of a rGO film annealed by a condition similar to (b). (e) AFM current image of (c). (d) and (f) magnified images of the samples in panels (c) and (e), respectively.
compared to pristine graphene also influence its low conductivity [32, 59]. Moreover, the existence of edge defects and residual oxygen functional groups lower the total conductivity [60]. After the reduction process, due to the elimination of oxygen-functional groups and new sp2 carbon atoms that are being introduced, the size of sp2 carbon clusters increases. Therefore, by connecting the graphene lattice domains, new electrical pathways among the sp2 domains form. Fig. 8a shows the comparison of the ID/IG ratio with the average ρs
values. We can observe that ρs value dropped along with the ID/IG ratio, suggesting that the increase in rGO sp2 cluster size caused by the new graphene synthesis and restoration of sp2 carbon lattice contribute to the efficient restoration of rGO conductivity [61]. Furthermore, in this study, we are able to synthesize thicker multilayer graphene at the boundary areas between the rGO flakes when increasing the reduction time of GO films in CH4 atmosphere. Although the quality of these graphene layers might not be very close to the mechanically exfoliated
Fig. 9. (a) Comparison of Raman spectra between the as-deposited GO and rGO films annealed in Ar atmosphere for different reduction times. Inset of (a) shows an AFM frictional image of the rGO film reduced in Ar atmosphere for 60 min. (b) Comparison of Raman spectra between the rGO film annealed in 3 different atmospheres. All rGO films were annealed for 60 min.
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Table 4 Average ρs values of the as-deposited GO and rGO films annealed for different reduction times in Ar atmosphere. Sample
Reduction time [min]
Sheet resistance, ρs [kΩ/sq]
GO rGO-Ar rGO-Ar rGO-Ar rGO-Ar rGO-Ar rGO-Ar
0 5 10 30 45 60 90
5.80 × 106 2.90 × 103 4.31 × 103 6.89 × 102 2.09 × 102 1.12 × 103 4.27 × 103
or metal-assisted CVD-grown graphene owing to their catalyst-free synthesis, they partially preserve their graphitic domains because the ρs values are decreased after these layers were grown. As a result, these multilayer graphene operates as the new electrical connection between the originally separated rGO flakes, as shown in the SEM image of Fig. 8b. Therefore, the high resistivity at the flake edges can be overcome, resulting in the high conductivity across the entire rGO films. Furthermore, Fig. 8c–f shows the conductive AFM images of the rGO film annealed by the condition similar to Fig. 8b. We can observe uniform distribution of current across the whole film, regardless of the difference between the original rGO flakes and the new graphene layers. However, we can still confirm the existence of hole-type defects of 100– 500 nm size in the rGO films that appear as the insulating regions in the current images. These large defects might be the result of the unrepairable dense defects that were introduced during the early stages of the oxidation process. Interestingly, the conductive properties of our continuous rGO films formed by the electrical connection are not affected by this type of defects. To confirm our assumptions in ‘Experiment 1’ and ‘Experiment 2’, we conducted thermal reduction experiment of GO films only in Ar or H2 [24,25] atmosphere. Fig. 8a shows comparison of Raman spectra of the rGO films annealed in Ar atmosphere for different reduction time. We can observe that, regardless of reduction time, there was almost no change in the intensity of G and 2D bands. These results suggest that, without CH4 flow, there was no restoration of carbon lattice defects on the graphene plane and no synthesis of graphene at the rGO edges, as confirmed by the AFM frictional images in the inset of Fig. 9a. To clarify the CH4 role, we plotted Raman spectra of the rGO films annealed in 3 different types of atmosphere for 60 min, as shown in Fig. 9b. There was a huge difference between the intensities of G and 2D bands of the rGO films, where the rGO films annealed in CH4 atmosphere exhibit much higher G and 2D band intensities. These results are in agreement with our assumption that rGO graphitic crystalline structure can be partially restored and increase sp2 cluster size by the annealing in CH4 atmosphere. Furthermore, ρs values of the rGO films annealed in H2 and Ar exhibit low electrical conductivity (~103 kΩ/sq) compared with the rGO films annealed in CH4 atmosphere, although they were annealed for the same reduction time, as shown in Table 4 and Table 5. For 60 min of reduction time, ρs value of the rGO films annealed in only Ar or H2 atmosphere is 3 orders of magnitudes higher than the ρs value of rGO films annealed in CH4 atmosphere. In the Raman spectra of Fig. 9b, we can locally fabricate less defective rGO films via the H2-assisted annealing by effectively removing the oxygen-functional groups, which leads to the healing of atomic level
Table 5 Comparison of the average ρs values of the as-deposited GO and rGO films annealed in CH4, H2 or Ar atmosphere for 60 min. Sample
Reduction time [min]
Sheet resistance, ρs [kΩ/sq]
GO rGO-CH4 rGO-Ar rGO-H2
0 60 60 60
5.80 × 106 3.07 1.12 × 103 2.54 × 103
defects [25]. However, the entire conductivity of the H2-annealed film is low compared with the rGO films annealed in CH4 atmosphere. We suggest that it is not sufficient to completely restore entire electrical conductivity of the rGO films without connecting the individual rGO flakes by forming continuous films. Furthermore, etching of the rGO flakes by H2 [24,45,46] also increases the resistivity at the graphene edges and affects the entire conductivity. 4. Conclusions In conclusion, towards the fabrication of large-area continuous rGO films, large amount of CH4 flow is needed for the new graphene synthesis at the GO edges due to the non-existence of catalyst. We realized that this phenomenon contradicted with the common CVD synthesis of graphene on transition metals. Besides, it is suggested that the original GO/rGO edges act as the starting points for the new graphene synthesis and we found that graphene continue to grow and stack in vertical direction from the GO/rGO edges, resulting in the formation of multilayer graphene only in the boundary areas between the flakes. Raman spectrum results demonstrate that the increase in the reduction time in CH4 atmosphere and the synthesis of multilayer graphene helped to partially restore the lattice defects and increase the size of graphene-like domains in rGO crystalline structure. However, there is still a challenge to completely heal the lattice defects and produce rGO films comparable to graphene derived from graphite, where the disruption of carbon-network might occur during the removal of functional groups from the GO plane owing to the high annealing temperature and long reduction time. Sheet resistance values show that restoration of electrical conductivity of rGO films is attributed to the formation of continuous rGO films. It has also been confirmed that the restoration of electrical conductivity is proportional to the amount of multilayer graphene synthesized and not solely from the reduction process, where this phenomenon has been confirmed by the thermal annealing in various atmospheres. We can certify that the formation of new conduction pathways between the separated rGO flakes through multilayer graphene formation and the restoration of lattice defects during the reduction process play important roles in the formation of highly conductive films. In spite of the existence of hole defects at the basal plane, the overall conductivity of the continuous rGO films are unaffected. Finally, this study displays a potential reduction method for mass production of large-area and continuous rGO films towards thin-film device applications. Author contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by Grant-in-Aid for scientific research (15K13361) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Authors thank Yokohama National University Instrumental Analysis Center (IA Center) for providing the XPS instrument. Authors also thank Daichi Yamaura and Ryosuke Kimura regarding the SEM, AFM measurements and useful discussions. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191.
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