Large-Scale Plasma Patterning of Transparent Graphene Electrode on

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Large scale plasma patterning of transparent graphene electrode on flexible substrates Ji Hye Kim, Euna Ko, Joonki Hwang, Xuan-Hung Pham, Joo Heon Lee, Sung Hwan Lee, VanKhue Tran, Jong-Ho Kim, Jin-Goo Park, Jaebum Choo, Kwi Nam Han, and Gi Hun Seong Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504443a • Publication Date (Web): 18 Feb 2015 Downloaded from http://pubs.acs.org on February 25, 2015

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Large scale plasma patterning of transparent graphene electrode on flexible substrates Ji Hye Kim,† Euna Ko,† Joonki Hwang,† Xuan-Hung Pham,† Joo Heon Lee,† Sung Hwan Lee,† Van-Khue Tran,† Jong-Ho Kim,‡ Jin-Goo Park,§ Jaebum Choo,† Kwi Nam Han,*,† and Gi Hun Seong*,†

†

Department of Bionano Engineering, Hanyang University, Ansan 425-791, South Korea

‡

Department of Chemical Engineering, Hanyang University, Ansan 425-791, South Korea

§

Department of Materials Engineering, Hanyang University, Ansan 425-791, South Korea

*Corresponding authors: Tel.: +82-31-400-5202 E-mail: [email protected] (G.H.S.), [email protected] (K.N.H.)

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ABSTRACT Graphene, a two-dimensional carbon material, has attracted significant interest for applications in flexible electronics as an alternative transparent electrode to indium tin oxide. However, it still remains a challenge to develop a simple, reproducible, and controllable fabrication technique for producing homogeneous large-scale graphene films and creating uniform patterns with desired shapes at defined positions. Here, we present a simple route to scalable fabrication of flexible transparent graphene electrodes using an oxygen plasma etching technique in a capacitively coupled plasma (CCP) system. Ascorbic acid-assisted chemical reduction enables the large-scale production of graphene with solution-based processability. Oxygen plasma in the CCP system facilitates the reproducible patterning of graphene electrodes, which allows controllable feature sizes and shapes on flexible plastic substrates. The resulting graphene electrode exhibits a high conductivity of 80 S·cm-1 and a transparency of 76%, and retains excellent flexibility upon hard bending at an angle of ±175° and after repeated bending cycles. A simple LED circuit integrated on the patterned graphene film demonstrates the feasibility of graphene electrodes for use in flexible transparent electrodes.


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INTRODUCTION Flexible and transparent electrodes have recently drawn a great deal of attention in various fields of electronics, such as flexible optoelectronics, field effect transistors, flexible batteries, wearable devices, and implantable medical devices.1-3 Despite rapid advances in flexible electronics, development of a robust technique for fabricating conductive, flexible, and transparent electrodes is still a key challenge. Thus far, metal oxide films, especially indium tin oxide (ITO), have been widely used as transparent electrodes, and these afford a high electrical conductivity and good optical transparency in the visual light range.4 However, the implementation of ITO as a flexible electrode has several limitations due to its mechanical brittleness (the micro-crack defects on ITO produced by repeated bending make it unsuitable for flexible devices), high cost owing to the limited amount of the element indium on earth, and the chemical instability of ITO in an acidic or basic environments. Therefore, it is highly desirable to develop alternative materials whose electrical and optical properties are sufficient for use in flexible electronics. Graphene, a single-atom-thick, two-dimensional sp2 carbon network material, has emerged as a rising star in material science.5,6 The outstanding electronic, optical, physical, and biocompatible properties of graphene have attracted tremendous interest in various applications including displays, composite materials, supercapacitors, transistors, energy storage/conversion, biochemical sensors, drug delivery, and healthcare.7-10 In particular, a thin layered graphene film providing high electrical conductivity, optical transparency, mechanical flexibility, and chemical stability, would have great promise for next-generation materials in a number of electronic devices, and would be capable of serving as a viable substitute for ITO.9 Prior to practical applications, however, it is imperative to develop a simple, reproducible, and controllable fabrication technique for producing homogeneous large-scale graphene films


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and creating uniform patterns with desired shapes at defined positions. Various synthetic methods such as chemical vapor deposition, mechanical exfoliation, epitaxial growth, and chemical oxidation/reduction have been explored to produce graphene films.11-14 Among these methods, the solution-based chemical approach is considered a relatively favorable method for large-scale production of graphene film due to several advantages including: (1) cost-effective and scalable production, (2) versatility in use for coating and patterning processes (e.g. spin coating, Langmuir Blodgett deposition, spray casting, vacuum filtration, and inkjet printing), and (3) easy functionalization for tuning properties of graphene. Solution-based chemical methods, which involve the chemical oxidation of graphite for conversion to graphene oxide (GO) and the subsequent reduction of GO to graphene, can permit stable dispersion in a variety of solutions and then facilitate the fabrication and manipulation of graphene for further applications by simple and inexpensive solution processes.15,16 To date, solution-based oxidation, known as the Hummers method, has been widely used for the synthesis of GO. The reduction of GO can be effectively accomplished either via chemical-deoxygenation using reducing agents or via thermaldeoxygenation by rapid heating to a high temperature. However, the thermal reduction route is not appropriate for flexible plastic substrates due to the necessity of extreme temperatures (~1000 °C).17 Over the last few years, a number of reducing agents, such as hydrazine/hydrazine-derivatives, hydroxylamine, sodium borohydride, and hydroquinone, have been investigated for GO reduction.18-21 Unfortunately, most of these reagents are too toxic to be implemented on a large scale. Moreover, the aggregation of rGO caused by their strong reducing ability is considered to be a serious drawback in solution-based processes. Hence, there is a strong demand for a non-toxic, effective, and environmentally-friendly alternative capable of being used for latge-scale rGO production in various applications.


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Plasma treatment is a technique widely used for the modification of graphene surfaces because it is clean, reliable, and practical for industrial processes.22,23 Ionized species generated by the plasma system break the sp2 lattice of graphene, leading to the introduction of functionalized groups on the graphene surface that allow for tuning of its electronic properties. Recently, several research groups have reported the use of plasma etching techniques incorporating inductively coupled plasma (ICP) systems or reactive ion etching (RIE) systems to etch graphene films.24-26 Although these methods do provide a rapid etch rate and precise resolution for graphene patterning, neither plasma system is suitable for flexible plastic substrates. The high plasma temperature generated in an ICP system can cause plastic substrates to melt, and plasma with high ion energies can also etch polymer substrates. Moreover, the remaining PR residue on graphene after ICP etching is considered as a serious problem affecting the electrical and optical properties of graphene film. Kwon et al. reported that the PR polymer reacted with the graphene layers due to the high-energy ICP etching, causing PR residue to remain on the surface of the graphene.24 This PR residue leads to decreased conductivity and transparency of graphene film. In the RIE system, both chemical and physical etching occur at the same time. The reactive ions vertically accelerated by a radiofrequency bias in RIE systems allow an anisotropic etching profile in the vertical direction. As a result of physical etching by high kinetic energy, polymer substrates can be damaged, and can exhibit severe changes in topology and surface roughness.27,28 Paul et al. reported that RIE technique could be applied for etching of both graphene and PMMA (poly(methyl methacrylate)) polymer.29 In contrast to the ICP and RIE systems, capacitively coupled plasma (CCP) systems may be preferable in use with polymer substrates due to their low ion energies and isotropic etching behavior. Relatively low ion energies do not cause damage on polymer surfaces, and homogeneous patterns can be achieved over large areas by isotropic etching with uniform plasma density in all directions.


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Herein, we present a simple route to scalable fabrication of flexible transparent graphene electrodes with controllable patterns via solution-based graphene production and oxygen plasma etching in a CCP system. The formation of graphene (reduced-GO or rGO) from GO was achieved through ascorbic acid (AA)-assisted reduction and solution-based processes. Subsequent oxygen plasma treatment provided reproducible fabrication of graphene patterns with controllable feature sizes and shapes. Electrical conductivity and optical transparency of as-prepared graphene electrodes were examined under various conditions of AA-assisted reduction and oxygen plasma treatment. Changes in physical and chemical properties of graphene were systematically characterized by UV-vis spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Moreover, mechanical bending tests were performed and a simple LED circuit was demonstrated to investigate the suitability of the patterned graphene electrodes for flexible electronic devices.

EXPERIMENTAL SECTION Materials Graphite, L-ascorbic acid (AA), sodium borohydride, hydroxylamine-hydrochloride, and potassium hydroxide were purchased from Sigma-Aldrich (St. Louis, MO, USA). Hydrazine monohydrate was purchased from TCI (Tokyo Chemical Industry, Tokyo, Japan). Polyethylene terephthalate (PET) substrates and anodic aluminum oxide (AAO) membranes were obtained from TOP NANOSYS Co. (Sung Nam, South Korea) and Whatman (Clifton, NJ, USA), respectively. Positive photoresist polymer (PR, AZ4620) was supplied by Clarivant Corporation (Somerville, NJ, USA).


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Preparation of rGO Films GO was prepared by the Hummers method (details are provided in the Supporting Information). To produce rGO, the reduction of GO was carried out with the assistance of AA as a reducing agent. GO suspension was treated by AA with a concentration ratio of 1:20. The reduction process was performed at 85 °C for 24 h without pH adjustment. The rGO suspension was then dialyzed against deionized water for two days. For the production of rGO films on PET substrates, as-prepared rGO suspensions were filtered using an AAO membrane with a 0.2 µm pore size. The AAO membrane underneath the rGO film was easily removed by using a 3 M NaOH solution, and the rGO film was subsequently transferred onto the PET substrate after neutralization with deionized water.

Patterning of rGO film and its characterization The patterning of the rGO film was accomplished by photolithography and oxygen plasma treatment. A positive PR was spin-coated on an rGO film at 1500 rpm for 1 min. After UV light (~365 nm) exposure on a PR-coated rGO film through the designed photomask, the rGO film was developed by AZ400K. Oxygen plasma treatments in the CCP system were performed at a 120 mTorr chamber pressure, 25-200 W power, and a substrate reflective frequency of 13.56 MHz for an exposure time of 1-5 min. After plasma treatments, the remaining PR on the rGO film was removed with an ethanol solution. The morphological and chemical changes in rGO films were investigated by field emission-scanning electron microscopy (FE-SEM, Hitachi Co. Ltd., S-4800, Japan), atomic force microscopy (AFM, Veeco Instruments, Inc., NanoScope IV, Santa Barbara, CA, USA), X-ray photoelectron


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spectroscopy (XPS, VG Microtech Inc., ESCA2000, West Sussex, UK), and Raman spectroscopy (Renishaw Ltd., Renishaw 2000 Raman, Gloucestershire, UK). The electrical and optical properties of rGO films were characterized by a four-point-probe system (Mitsubishi Chemical Corp., MCP-T610, Japan) and UV–vis spectroscopy (Mecasys Co. Ltd., OPTIZEN 2120UV, South Korea), respectively.

Mechanical bending test and LED demonstration on the patterned rGO electrode For characterization of electrical behavior upon mechanical bending, the current responses from the rGO electrode were examined using an electrochemical analyzer (CH Instruments, Inc., CHI660C, Austin, TX, USA). With given bending angles ranging from -175° to 175°, the change in current response was measured at an applied voltage of 0.1 V. The effect of repeated bending on the conductivity of the rGO electrode was also explored after repeated bending cycles at an angle of ±90°. To investigate the role of rGO films as flexible transparent electrodes, a LED circuit was integrated on a patterned rGO electrode with a width of 1 mm and a length of 18 mm. Ag paste was coated on opposite terminals of rGO electrodes to provide electrical contact pads. LED demonstration was performed at a turn-on voltage of 2 V.

RESULTS AND DISCUSSION To produce large-scale graphene, natural graphite was oxidized using the Hummers method and was exfoliated as GO into an aqueous solution by ultra-sonication. The obtained GO with numerous oxygen-containing groups (carboxyl, epoxy, carbonyl, and hydroxyl) exhibited a


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stable dispersion in water, but conductivity was lost due to the change in electronic properties by breaking the sp2 structure of the graphene. To restore its conductivity, the de-oxygenation of GO can be accomplished by chemical reduction methods using hydrazine, hydroxylamine, sodium borohydride, or hydroquinone as the reducing reagent. However, most of these reagents are toxic, explosive, and harmful. Moreover, the aggregation of rGO that results due to their strong reducing ability hinders further use in solution-based processes. To avoid these obstacles, AA (Vitamin C), which is a non-toxic and mild reductive reagent, was employed as an environmentally-friendly reducing agent in this work. The effect of concentration ratio of GO-to-AA on GO reduction was investigated in a GO suspension (0.01 mg·ml-1) with different concentration ratios of GO:AA ranging from 1:10 to 1:500. AA-assisted reduction was carried out at 85 °C for 24 h without any surfactants or stabilizers. After AA-assisted reduction, a color change of the GO suspension from light brown to black was observed (Figure S1). This color change clearly revealed that the regraphitization of the exfoliated GO was effectively achieved by AA. Particularly, the resulting rGO suspension treated by AA with a concentration ratio of 1:20 showed a blackcolored, stable dispersion for months, whereas the precipitation of rGO occurred at a high concentration ratio of more than 1:50 due to increased incompatibility with the aqueous solution. Next, the influences of temperature and pH on GO reduction were examined in terms of the change in conductivity and transparency of the rGO film. The rGO suspensions were prepared by AA-assisted reduction (1:20 ratio) at various temperatures (from 25 to 85 °C) and pH values (from 3.5 to 8.5) for 24 h, followed by vacuum filtration to fabricate rGO films on PET substrates. The conductivity and transparency of as-prepared rGO films were measured using a four-point-probe and UV–vis spectroscopy, respectively. As shown in


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Figure S2, the reaction temperature significantly affected the conductivity of prepared rGO films, whereas the pH value had an almost negligible impact. The conductivity of rGO films drastically increased as the temperature increased up to 85 °C, but the aggregation of the rGO suspension occurred above 100 °C, thereby reducing the uniformity and conductivity of the rGO film (data not shown). The increase in conductivity at high temperatures is attributed to the chemical mechanism of AA-assisted reduction. Specifically, the formation of rGO is accomplished by thermal elimination of the intermediate resulting from chemical reaction between GO and AA, which is accompanied by the release of dehydro-AA; this is favored at higher temperatures, resulting in increased conductivity.30 Figure 1 shows AFM images of GO and rGO produced by AA-assisted reduction with a concentration ratio of 1:20 at 85 °C without pH adjustment. The thickness of rGO layers measured from the height profile (thickness = average height – average baseline) was about 0.7 nm, which was consistent with a graphene monolayer.31,32 In contrast, the thickness of GO sheets was measured to be ~1.0 nm due to oxygen-containing groups decorated around the original graphene plane. In addition, we compared the reduction efficiency of various reductants (e.g. sodium borohydride, hydroxylamine-hydrochloride, potassium hydroxide, and hydrazine monohydrate) with AA-assisted reduction in terms of conductivity and transparency (Figure S3). The rGO suspension prepared by AA reduction showed a stable dispersion in aqueous solution, which permitted solution-based processability for thin rGO film coating by vacuum filtration. The as-prepared rGO film showed a high conductivity of 70-100 S·cm-1 with a transparency of 70-80%. In contrast, the conductivity of rGO films obtained by other reductants was less than half of that of AA even though their reduction was performed under the optimum conditions of the AA-assisted reduction (Figure S3B). This


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result indicates that AA-assisted reduction is apparently comparable to other reduction agents for the re-graphitization of GO. Figure 2A shows a schematic process for the patterning of rGO films using the oxygen plasma treatment in CCP system. Reproducible thicknesses of coated rGO layers can be achieved by controlling the concentration of a rGO suspension using a vacuum filtration method. With the aid of conventional photolithography, the oxygen plasma treatment enabled precise etching at desired positions with high resolution. During the oxygen plasma treatment, the rGO layers underneath the PR polymer were protected from plasma etching while the rGO layers on exposed areas were destroyed and finally removed from the substrate surface. Optical images of patterned rGO films on PET substrates are shown in Figure 2B. The patterned rGO exhibited no changes in conductivity and transparency before and after oxygen plasma treatment, whereas the etched area became non-conductive and more transparent. The resulting rGO patterns were consistent with that of a designed mask, and could be applied as an optically transparent electrode with high transparency, high conductivity, and good mechanical flexibility. In addition, the conductivity gradually increased from 5 S·cm-1 to 133 S·cm-1 as the thickness of the coated rGO increased from 3 nm to 30 nm, but the transparency decreased from 85 to 45% (Figure S4). The best electrical and optical performance (conductivity of ~80 S·cm-1 and transparency of ~76% at 550 nm) was obtained at a thickness of 10 nm. Considering that the thickness of coated rGO layers affects both conductivity and transparency, a thickness of 10 nm was used for transparent electrodes. A comparison of the conductivity and transparency for other techniques is shown in Table S1. To investigate the effect of oxygen plasma treatment on the conductivity and transparency of rGO films, oxygen plasma treatment was carried out under various conditions with different combinations of plasma power (50-200 W) and treatment time (1-5 min). In Figure


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3A, the conductivity of the rGO film decreased from 77 S·cm-1 to 0.1 S·cm-1 as the plasma power and time increased. When oxygen plasma treatment was performed at 100 W for 5 min or at 200 W for more than 3 min, it was impossible to measure the conductivity because it was below the measurement range of the instrument (< 0.001 S·cm-1). The drastic drop in conductivity at the above conditions can be ascribed to the lack of a conductive percolation path. The percolation threshold is defined as the critical value at which a continuous connected network is formed for the transport of electrons through the percolation pathway. Below this value, the conductivity decreases drastically, as shown in the experimental results mentioned above. Thus, the destruction of rGO layers driven by oxygen plasma eliminates conductive percolation pathways resulting in a drastic decrease in conductivity. Figure 3B shows the changes in the conductivity and transparency of rGO films at a constant plasma power of 200 W. As the plasma treatment time increased up to 5 min, the transparency of the rGO film was improved from 76 to 90% while the conductivity decreased below the limit of measurement (the transparency of the PET film itself was 92%). In consideration of the applications for flexible transparent electrodes, plasma treatment at 200 W for 5 min was chosen as the patterning condition to allow high performance in both conductivity and transparency. SEM images in Figure 4 demonstrate the morphological changes in the rGO film before and after oxygen treatment. In a pristine rGO film, the homogeneous network of rGO layers appeared to be clear. The puckered morphology of the rGO film indicates the formation of a network of interconnected rGO layers. However, significant morphological changes were observed after oxygen plasma treatment at a power of 200 W for 5 min. Here, most of the rGO disappeared from the substrate due to oxygen plasma etching, and the little remaining residue seemed to be amorphous carbon with a clumped globular structure. The conductive


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network between rGO layers was entirely destroyed, and, consequently, conductivity was lost. For more detailed characterization of chemical composition changes according to plasma treatment, XPS was employed (Figure 5). The XPS survey scans highlight the disappearance of C1s peaks. Oxygen radicals generated from the plasma damage the rGO, leading to the release of volatile species such as CO and CO2 from the damaged rGO layers. Figures 5C and D are the deconvoluted XPS spectra of C1s (~284 eV) used to probe the chemical composition changes of the rGO surface more clearly, which involve four different peaks that correspond to carbon atoms with different chemical valences. The peaks centered at 284, 285, 287, and 288 eV were assigned to the carbon atoms of C-C (sp2 carbon), C-O (epoxy and alkoxy), C=O (carbonyl), and O=C–O (carboxyl), respectively. We noted that the content of sp2 carbon decreased from 63 to 52% after plasma treatment while the content of carbon containing oxygen groups increased from 37 to 48%. This observation indicates that oxygen plasma breaks sp2 domains of the rGO and introduces different oxygen groups on rGO defect sites, resulting in rGO etching with conversion to amorphous carbon, as shown in the SEM images. In Figure 6, structural changes in the rGO film by plasma treatment were characterized by Raman spectroscopy. Raman spectra of the rGO film showed two dominant peaks at 1590 cm-1 (G band) and 1352 cm-1 (D band) corresponding to first-order scattering of the E2g phonon of sp2 carbons and a breathing mode of k-point phonons of A1g symmetry related to disorder in sp2-hybridized carbons (sp2 rings), respectively.33,34 As the plasma exposure time increased, the G and D bands gradually decreased compared to untreated rGO (Figure 6A). This implies a decrease in the coverage of rGO on the substrate resulting from oxygen plasma etching. In addition, the structural defects and disorder of rGO can be monitored by the ratio of the peak intensities of the D and G bands (ID/IG). Specifically, the ID/IG ratio indicates the


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amount of disorder that is directly related to the average size of the crystalline regions (La). In our study, the ID/IG ratio decreased linearly with the plasma exposure time, as shown in Figure 6B. This feature of the Raman spectra agrees with the amorphization stage associated with the structural change from nanocrystalline graphite to sp3 amorphous carbon according to a three stage classification for the structural disorder in graphitic materials.35-37 In this stage, Ferrari and Robertson noted that the ID/IG ratio is proportional to La2 and the number of sp2 ordered rings: I(D)/I(G) = C(λ)·La2, where C(λ) denotes a constant at the particular excitation wavelength (λ) used in Raman measurements (C(514 nm) = ~0.55). The decrease in the ID/IG ratio in Figure 6B is therefore indicative of decrease in the size of sp2 domains and number of ordered rings on rGO layers with increasing plasma treatment. These results reflect the destruction of a conductive network between rGO layers as observed in the SEM images. To determine the suitability of rGO electrodes in flexible applications, the electrical behavior was tested during mechanical bending. Figure 7A shows the influence of mechanical bending on current response from the rGO electrode. The current responses were measured for inward (from 0 to 175°) and outward (from 0° to -175°) bending angles at an applied voltage of 0.1 V. The variation of current responses (∆I = I(θ) – I(0)) appeared to be negligible in the bending angle range from -100° to 100°, and exhibited a slight decrease of less than 1.5% at an extremely hard bending angle of ±175°. After a hard bending, the recovery rate of the current response was about 98% compared to the original value. This observation probably resulted from changes in the surface morphology of rGO layers caused by mechanical deformation and reorientation of stacked rGO layers. Figure 7B shows the effect of repeated bending on the conductivity of the rGO electrode, which was measured after repeated bending cycles at an angle of ±90°. There was no significant change in


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conductivity of the rGO electrode with increasing bending repetition up to 50 cycles. These results reveal the high electrical performance of rGO electrodes during bending, indicating that they are suitable for use as flexible transparent electrodes in displays and optoelectronics. In addition, the suitability of the patterned rGO electrodes for electronic devices was demonstrated by a simple LED circuit integrated on rGO electrodes, as shown in Figures 7C and D. The rGO electrodes consisted of a series of parallel patterns that were connected with a positive and negative terminal. The LEDs were integrated on patterned rGO electrodes, and Ag paste was used to form contact pads on opposite terminals of rGO electrodes. In Fig. 7D, the prepared LED circuit was operated at a turn-on voltage of 2 V. This LED circuit demonstration indicated that the patterned rGO electrodes can be used as transparent electrodes to replace ITO for flexible electronics. Moreover, it revealed the effectiveness of the CCP plasma technique for rGO patterning and the feasibility of patterned rGO electrodes for real applications in large-scale flexible electronics.

CONCLUSIONS We report a facile approach for scalable fabrication of conductive transparent rGO electrodes on flexible substrates. AA-assisted chemical reduction enabled the large-scale production of rGO from natural graphite, and an oxygen plasma etching technique in a CCP system produced well-defined rGO electrode patterns from solution-processed rGO films. The resulting graphene electrode exhibited a high conductivity of 80 S·cm-1 with a transparency of 76% at a thickness of 10 nm. There was no obvious change in the current response and conductivity upon hard bending at an angle of ±175° and repeated bending cycles. A simple LED circuit integrated on the patterned rGO film demonstrated the possibility of rGO electrodes for use in flexible transparent electrodes. This method holds promise for large-


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scale fabrication of rGO electrode patterns on flexible substrates and for applications in flexible electronics.

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2008-0061891 and 2013R1A1A2054887).

ASSOCIATED CONTENT Supporting Information Influence of concentration ratio, temperature and pH on the AA-assisted reduction of GO, comparison between AA and other reducing agents, change in conductivity and transparency depending on the thickness of rGO film, and SEM images of rGO films according to oxygen plasma treatment, and table for a comparison of the conductivity and transparency for other techniques. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G.H.S.), [email protected] (K.N.H.) These corresponding authors contributed equally.


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Notes The authors declare no competing financial interest.


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Figure 1. AFM images of (A) GO and (B) rGO layers. The insets show the height profile of each region taken from the dashed line. The scale bars are 300 nm.


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Figure 2. (A) Schematic illustration for fabrication of rGO electrode patterns on PET substrate. (B) Optical images of the patterned rGO electrodes obtained by oxygen plasma treatment at 200 W for 5 min. The scale bars are 500 µm.


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Figure 3. (A) Effect of oxygen plasma treatment on the conductivity and transparency of rGO films. Graphene films were treated with oxygen plasma under different combinations of plasma power (25-200 W) and exposure time (1-5 min). (B) Changes in conductivity and transparency according to plasma treatment time at a constant power of 200 W.


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Figure 4. SEM images of rGO films treated by oxygen plasma at a power of 200 W for (A) 0 min, (B) 1 min, (C) 3 min, and (D) 5 min. The scale bars are 300 nm.


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Figure 5. XPS survey spectra of rGO films before (A), (C) and after (B), (D) oxygen plasma treatment at 200 W for 5 min.


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Figure 6. (A) Raman spectra of rGO films after plasma treatment at 200 W for varied exposure times (0-5 min). (B) Changes in peak intensity ratio of the D and G bands (ID/IG) with increasing oxygen plasma exposure time.


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Figure 7. (A) Variation of current responses from the rGO electrode upon mechanical bending. (B) Changes in conductivity due to repeated bending cycles. Optimal image of the LED circuit integrated on the patterned rGO electrode in the off-state (C) and the on-state (D).


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TOC


Large-Scale Plasma Patterning of Transparent Graphene Electrode on

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