Low-temperature synthesis of large-area graphene

Low-temperature synthesis of large-area graphene-based transparent conductive films using surface wave plasma chemical vapor deposition Jaeho Kim, Masatou Ishihara, Yoshinori Koga, Kazuo Tsugawa, Masataka Hasegawa et al. Citation: Appl. Phys. Lett. 98, 091502 (2011); doi: 10.1063/1.3561747 View online: http://dx.doi.org/10.1063/1.3561747 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v98/i9 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 98, 091502 共2011兲

Low-temperature synthesis of large-area graphene-based transparent conductive films using surface wave plasma chemical vapor deposition Jaeho Kim, Masatou Ishihara, Yoshinori Koga, Kazuo Tsugawa, Masataka Hasegawa,a兲 and Sumio Iijima Nanotube Research Center, National Institute of Advanced Industrial Science and Technology, Tukuba Cetral 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

共Received 24 December 2010; accepted 8 February 2011; published online 2 March 2011兲 We present a low-temperature 共300– 400 ° C兲, large-area 共23 cm⫻ 20 cm兲 and efficient synthesis method for graphene-based transparent conductive films using surface wave plasma chemical vapor deposition. The films consist of few-layer graphene sheets. Their transparency and conductivity characteristics make them suitable for practical electrical and optoelectronic applications, which have been demonstrated by the proper operation of a touch panel fabricated using the films. The results confirm that our method could be suitable for the industrial mass production of macroscopic-scale graphene-based films. © 2011 American Institute of Physics. 关doi:10.1063/1.3561747兴 Graphene-based films are potential candidates for transparent conductive films for next-generation electrical and optoelectronic devices and various other applications due to their high electrical conductivity, as well as chemical and physical stability.1–10 For the industrial mass production of graphene-based films, a low-temperature, rapid, and largearea synthesis is a key technology. A low-temperature process is particularly essential for the Si-based device industry and would pave the way for the direct synthesis of graphenebased films onto glass and plastic substrates. Two methods for synthesizing graphene-based films have been previously proposed; the thermal chemical vapor deposition 共CVD兲 method on metal surfaces1–4 and the chemical reduction method of graphite oxides.5–9 These methods could be scaled up in synthesizing a larger film size. However, the former method is restricted to a high deposition temperature limit of around 1000 ° C and the latter method requires timeconsuming procedures and complicated liquid waste treatment. Plasma CVD is also expected to be a useful method for graphene-based film synthesis. Reactive gas-species 共radicals兲, produced by high-density plasmas, permit lowtemperature, and rapid synthesis of graphene-based films. Several approaches to graphene synthesis using microwave plasma CVD at high gas pressures 共1 atm and 5.3 kPa兲 have been reported so far.11,12 However, it cannot yet be used for the industrial mass production of graphene-based films because the synthesis temperatures are still high and the plasma discharge characteristics in the previous works limit the scale-up of the film synthesis area. Recently, we developed a large-area surface wave plasma 共SWP兲 CVD apparatus with a plasma area of 40 cm⫻ 60 cm by adopting an array configuration of microwave slot antennas for synthesizing carbon nanomaterials.13 In the present work, for synthesizing graphene-based films, we utilized the original large-area SWP and operated at gas pressures less than 5 Pa. Such a low-pressure process allows to keep a substrate temperature lower due to low neutral gas temperatures and also uniform CVD growth over large areas due to the enhanced diffusion of plasma. SWP is able to a兲

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0003-6951/2011/98共9兲/091502/3/$30.00

provide high-density plasmas and radicals and has relatively low electron temperature 共below 2 eV兲 and also low plasma space potential 共below 10 V兲 in bulk region even at the low gas pressures. As a result SWP can reduce ion bombardment on the substrate surface.13,14 SWP generated at such a low gas pressure exhibits outstanding properties for the lowtemperature and rapid synthesis of graphene-based films. Here we show the potential of the SWP-CVD method for the industrial mass production of macroscopic-scale graphenebased films. Various metals such as ruthenium 共Ru兲,15 iridium 共Ir兲,16 nickel 共Ni兲,1,2,17,18 and copper 共Cu兲1,3,4 have been used as substrate materials for graphene synthesis. Among these metals, Cu is useful for the mass production of large-area graphene films since large-area Cu foils are commercially available and inexpensive compared with the other metal foils. Aluminum 共Al兲 is another attractive substrate material for graphene CVD provided it can tolerate the temperature during our CVD process. We therefore examined the formation of graphene-based films on both Cu 共30 ␮m thick兲 and Al 共12 ␮m thick兲 foils. The polycrystalline Cu and Al foils were treated to clean their surfaces by using Ar/ H2 plasmas at 5 Pa for 20 min before the CVD process. After the treatment, we turned off the microwave generators, evacuated the discharge chamber to base pressure of 10−3 Pa, introduced a reaction gas mixture into the discharge chamber. We then performed CVD using these foils as substrates at 3 or 5 Pa with microwave powers of 3 to 4.5 kW for each microwave generator for various growth times ranging from 30 to 180 s. During the CVD process, the temperatures of the foils were measured by using a thermocouple attached to the foil surface. The deposited films were characterized by a Raman spectrometer 共XploRa, Horiba兲, with a Raman excitation wavelength of 638 nm and beam spot size of 1 ␮m. To clarify the characteristics of deposited graphene films as a transparent electrode, optical transmittance 共U-4100, Hitachi兲 and sheet resistance 共MCP-T600, Mitsubishi Chemical Co., Ltd. 兲 were also measured. The measurements were performed on the films which were directly transferred to glass plates after the Cu substrates were etched away in FeCl3 aqueous solution and the films were washed in de-ionized

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FIG. 1. 共Color online兲 Raman spectra 共638 nm laser, 1 ␮m spot size兲 of graphene-based films deposited at substrate temperatures below 400 ° C by SWP-CVD. 共a兲 Raman spectrum of a typical graphene-based film deposited on Cu foil 共CVD conditions: 5 Pa, CH4 / Ar/ H2 = 30/ 20/ 10 SCCM, 3 kW per a MW generator, 30 s兲. 共b兲 Raman spectrum of a graphene-based film deposited on Al foil 共CVD conditions: 3 Pa, CH4 / Ar/ H2 = 30/ 20/ 10 SCCM, 4 kW per a MW generator, 180 s兲. 共c兲 Substrate temperature profile during the Ar/ H2 plasma treatment and the plasma CVD for the synthesis of the film on Al foil shown in Fig. 1共b兲.

water.1 To measure the spatial distribution of sheet resistance and to fabricate electrodes of a touch panel, the graphenebased films deposited on Cu foils were transferred to acrylic plates. We examined the formation of the graphene-based films using CH4 gas mixed with Ar and/or H2. Although graphene can be grown using CH4 alone in our plasma CVD process, the addition of Ar improves the stability and uniformity of the plasma at low gas pressure. The addition of H2 is expected to improve the quality of the films because hydrogen is known to selectively etch amorphous carbon defects.19,20 Although the optimal gas mixture has not been fully elucidated for high-quality films under our CVD conditions, we obtained graphene-based films with reaction gas mixtures of CH4 / Ar= 30/ 10 SCCM and CH4 / Ar/ H2 = 30/ 20/ 10 SCCM 共standard cubic centimeters per minute兲 in this experiment. The reaction gas mixtures enable the synthesis of graphene-based films on Cu and Al foils at substrate temperatures lower than 400 ° C. Figure 1共a兲 shows a Raman spectrum of a typical graphene-based film deposited on Cu. The spectrum shows three main peaks at 1326 cm−1, 1578 cm−1, and 2657 cm−1, corresponding to D, G, and 2D peaks, respectively. The D⬘ shoulder at 1612 cm−1 is also found in the spectrum. The position and shape of the 2D peak are known to distinguish the number of graphene layers.21–23 In addition, the intensity ratio of the 2D to G peaks provides a good correlation with the number of graphene layers.21 The 2D peak shown in Fig. 1共a兲 has a full width at half maximum of 37 cm−1, which is well fitted by a single Lorentzian line. The peak height ratio of the 2D to G

Appl. Phys. Lett. 98, 091502 共2011兲

FIG. 2. 共Color online兲 Characteristics of graphene-based films as transparent electrodes. 共a兲 Optical transmittance of a graphene-based film transferred on a glass plate. The discontinuity at around 800 nm in the curve arises from the different sensitivities of the switching detectors. The inset shows a picture of the film with 81% transparency. 共b兲 Plot of sheet resistance vs optical transmittance 共average between 400 and 800 nm兲 of the films obtained by SWP-CVD with various CVD conditions.

peaks is 3.4 共the integrated intensity ratio of the 2D to G peaks is 4.9兲. Figure 1共a兲 suggests that Raman peaks are assigned to the peaks of few-layer graphene in the deposited film. The high-intensity D peak and low-intensity D⬘ shoulder in the Raman spectrum shown in Fig. 1共a兲 are attributed to the edges and the boundaries of the flakes,24–26 which suggests that the film contains submicrometer sized flakes of graphene-based materials. From these considerations, we conclude that the deposited film is the accumulation of fewlayer graphene films. Our plasma CVD process permits the formation of graphene-based films on Al foils without any thermal damage. Figure 1共b兲 shows a Raman spectrum of a graphenebased film deposited on Al foil. During the plasma CVD, the substrate temperature was heated from 157 to 317 ° C as shown in Fig. 1共c兲. These results indicate the advantageous low-temperature synthesis characteristics of SWP-CVD compared with the conventional thermal CVD of graphene on Cu substrates. Figure 2共a兲 shows the optical transmittance of a graphene film transferred on a glass plate. The film has a transmittance of 81% 共average between 400 and 800 nm兲. Figure 2共b兲 plots the sheet resistance versus optical transmittance of the deposited films under our CVD conditions. The sheet resistance and optical transmittance of the films are 2.2 to 45 k⍀ per square and 78% to 94%, respectively. The electrical conductivity and optical transmittance of our films are lower than those of high-quality graphene films deposited by thermal CVD.2 Recently, high-quality graphene films not containing the D peak in Raman spectra were synthesized on a Ni substrate using remote plasma CVD.27 The report sug-


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FIG. 3. 共Color online兲 Synthesis of large-area graphene-based films with an area of 23 cm⫻ 20 cm using SWP-CVD 共CVD conditions: 3 Pa, CH4 / Ar/ H2 = 30/ 20/ 10 SCCM, 4.5 kW per a MW generator兲. 共a兲 Cu film after plasma CVD. 共b兲 Transferred film on an acrylic plate deposited for 180 s. 共c兲 Transferred film on an acrylic plate deposited for 90 s. 共d兲 Spatial distribution of the sheet resistance of the graphene-based film transferred on an acrylic plate shown in Fig. 3共b兲.

gests that the quality of our films can be improved by optimizing the plasma CVD conditions. To examine the quality of our films for practical applications, we fabricated a prototype of a capacitive touch panel using large-area films deposited by SWP-CVD as transparent electrodes. Large-area graphene-based films were deposited on Cu foils with an area of 23 cm⫻ 20 cm and their spatial uniformity of sheet resistance was measured. Figure 3共a兲 shows a Cu foil after the plasma CVD process. Figures 3共b兲 and 3共c兲 show the films transferred on acrylic plates deposited at a gas pressure of 3 Pa for 180 s and 90 s, respectively. Figure 3共d兲 shows the spatial distribution of sheet resistance of the film transferred on an acrylic plate shown in Fig. 3共b兲. The sheet resistance is 1.0 to 4.1 k⍀ per square over the area of 23 cm⫻ 20 cm. As far as we know, this is the first report on the spatial uniformity of a large-scale graphenebased film. Figure 4 shows a photograph of the touch panel working properly. It was confirmed that the touch panel has sensitivity to be operated by finger touch. This demonstration indicates that the graphene-based films, synthesized by SWP-CVD, are promising materials for transparent conductive electrodes.

FIG. 4. 共Color online兲 Picture of a fabricated touch panel using the graphene-based transparent conductive films deposited by SWP-CVD. The blue and the red LEDs are lighted properly by finger touch.

In conclusion, we have deposited large-area transparent conductive films based on graphene using a SWP-CVD method on Cu and Al foils at substantially lower temperatures 共below 400 ° C兲 than existing techniques for graphenebased-film synthesis. The films exhibit adequate optical and electrical properties for use as transparent conductive films. A touch panel fabricated using the graphene-based films worked properly. Our low-temperature synthesis method using plasma CVD is suitable for industrial automated fabrication processes for various applications. The synthesis method presented in this paper could be expanded to the continuous production of large-area graphene-based films using the array configuration of microwave plasma launchers and the roll-to-roll manufacturing method. 1

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