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Fabrication of graphene-based flexible devices utilizing a soft lithographic patterning method
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 285302 (http://iopscience.iop.org/0957-4484/25/28/285302) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 130.133.8.114 This content was downloaded on 24/04/2017 at 12:24 Please note that terms and conditions apply.
You may also be interested in: Reduced graphene oxide nanoshells for flexible and stretchable conductors Wen-Shuai Jiang, Zhi-Bo Liu, Wei Xin et al. Defect-free functionalized graphene sensor for formaldehyde detection Xiaohui Tang, Nathalie Mager, Beatrice Vanhorenbeke et al. Enhancing gas sensing properties of graphene by using a nanoporous substrate Cheol-Soo Yang, Ather Mahmood, Bongseock Kim et al. Engineering electrical properties of graphene: chemical approaches Yong-Jin Kim, Yuna Kim, Konstantin Novoselov et al. Microorganism mediated synthesis of reduced graphene oxide films Y Tanizawa, Y Okamoto, K Tsuzuki et al. Programmable high crystallinity carbon patterns Xuewen Wang, Hong Wang, Yang Gu et al. Ultrafast and sensitive room temperature NH3 gas sensors based on chemically reduced graphene oxide Nantao Hu, Zhi Yang, Yanyan Wang et al. Sodium deoxycholate functionalized graphene and its composites with polyvinyl alcohol Lanwei Wang, Ruijuan Liao, Zhenghai Tang et al. Fabrication of highly oriented reduced graphene oxide microbelts array for massive production of sensitive ammonia gas sensors Jia Zhang, Rongfu Zhang, Xiaona Wang et al.
Nanotechnology Nanotechnology 25 (2014) 285302 (6pp)
doi:10.1088/0957-4484/25/28/285302
Fabrication of graphene-based flexible devices utilizing a soft lithographic patterning method Min Wook Jung1,2,4, Sung Myung1,4, Ki Woong Kim1, Wooseok Song1, You-Young Jo3, Sun Suk Lee1, Jongsun Lim1, Chong-Yun Park2 and Ki-Seok An1 1
Thin Film Materials Research Group, Korea Research Institute of Chemical Technology, Daejeon 305-543, Republic of Korea 2 Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea 3 National Academy of Agricultural Science, Rural Development Administration, Suwon 441-707, Republic of Korea E-mail: [email protected] Received 6 January 2014, revised 8 April 2014 Accepted for publication 20 May 2014 Published 27 June 2014 Abstract
There has been considerable interest in soft lithographic patterning processing of large scale graphene sheets due to the low cost and simplicity of the patterning process along with the exceptional electrical or physical properties of graphene. These properties include an extremely high carrier mobility and excellent mechanical strength. Recently, a study has reported that single layer graphene grown via chemical vapor deposition (CVD) was patterned and transferred to a target surface by controlling the surface energy of the polydimethylsiloxane (PDMS) stamp. However, applications are limited because of the challenge of CVD-graphene functionalization for devices such as chemical or bio-sensors. In addition, graphene-based layers patterned with a micron scale width on the surface of biocompatible silk fibroin thin films, which are not suitable for conventional CMOS processes such as the patterning or etching of substrates, have yet to be reported. Herein, we developed a soft lithographic patterning process via surface energy modification for advanced graphene-based flexible devices such as transistors or chemical sensors. Using this approach, the surface of a relief-patterned elastomeric stamp was functionalized with hydrophilic dimethylsulfoxide molecules to enhance the surface energy of the stamp and to remove the graphene-based layer from the initial substrate and transfer it to a target surface. As a proof of concept using this soft lithographic patterning technique, we demonstrated a simple and efficient chemical sensor consisting of reduced graphene oxide and a metallic nanoparticle composite. A flexible graphene-based device on a biocompatible silk fibroin substrate, which is attachable to an arbitrary target surface, was also successfully fabricated. Briefly, a soft lithographic patterning process via surface energy modification was developed for advanced graphene-based flexible devices such as transistors or chemical sensors and attachable devices on a biocompatible silk fibroin substrate. Significantly, this soft lithographic patterning technique enables us to demonstrate a simple and efficient chemical sensor based on reduced graphene oxide (rGO), a metallic nanoparticle composite, and an attachable graphene-based device on a silk fibroin thin film. S Online supplementary data available from stacks.iop.org/NANO/25/285302/mmedia Keywords: soft lithography, graphene, flexible devices (Some figures may appear in colour only in the online journal) 4
These authors contributed equally.
0957-4484/14/285302+06$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
M W Jung et al
Nanotechnology 25 (2014) 285302
∼30 nm. Hydrazine is the most common and effective reducing agent used to convert GO to reduced GO (rGO). As a result, the GO or GO-PdNP composite on the substrate was reduced overnight by hydrazine vapor at 80 °C, as described in previous studies [6–8]. The rGO and rGOPdNP composite have a black-colored and hydrophobic surface, and the rGO has a smaller surface energy than GO before reduction due to the hydrophobicity of the rGO surface. We optimized the surface energy of rGO and the rGO-PdNP composite as well as the adhesion between graphene-based materials and the initial substrate by the reduction of GO. In figure 1(c), hydrophilic molecules such as DMSO were used to enhance the adhesive force between the rGO (or rGO-PdNP) layer and the surface of the elastomeric mold. DMSO diluted in deionized (DI) water (DMSO:DI water = 1:2 (v:v)) was vaporized and coated on the PDMS surface at a level 5 cm above the diluted DMSO-water solution at 200 °C for 1 min. for the uniform deposition on the PDMS surface. When the PDMS mold was contacted with the layer of the graphene-based materials such as rGO or the rGO-PdNP composite for a few minutes, the graphene-based material was removed from the initial substrate and transferred to the PDMS stamp, which has a higher surface energy than the SiO2 substrate. When PDMS with the graphene-based materials contacted the surface of the target substrate for 2 mins. at 70 °C, the surface energy of the PDMS was restored. The graphene-based materials were subsequently detached from the PDMS surface (figure 1(e)). Finally, the source and drain electrodes were fabricated on the top of the graphenebased material patterns via e-beam evaporation of metals through a shadow mask. A scanning electron microscopy (SEM) image and the Raman spectrum of the rGO patterns obtained using the soft lithographic patterning method are shown in figure 2(a). As shown in the SEM image, we could obtain uniform rGO line electrodes with a high resolution (∼8 μm) width compared to a previous study, in which it was reported that rGO pieces were patterned without using any assistive compounds [1]. Here, the dark area in the SEM image of the rGO patterns represents the remaining rGO patterns after the removal of the rGO pieces. The bright area corresponds to the exposed SiO2 surface. The G-band in the Raman spectrum (right image of figure 2(a)) is located at 1580 cm−1, which originates from a first order Raman scattering process and corresponds to the in-plane vibration of sp2-bonded carbon atoms. The Raman peak at 1350 cm−1 is associated with defects in graphene, and it is usually referred to as the disorder-induced D-peak. The two-dimensional peak at 2680 cm−1 originates from a two phonon double resonance Raman process and is approximately twice the frequency of the D-peak. The Raman intensity map of the G-band of the rGO patterns is shown in figure 2(b). The differences between the rGO patterns and the bare SiO2 surface are obvious. It can be clearly seen that the bright area in the left optical microscopy image corresponds to rGO patterns remaining on the SiO2 substrate. These results show that our method can be readily utilized for the
1. Introduction In the past decade, various devices based on graphene have been developed due to its excellent electrical and physical characteristics, such as a high carrier mobility, thermal conductivity, mechanical flexibility, and optical transparency [1, 2]. However, a prerequisite for the development of graphene-based electronic devices is a reliable patterning technique [3]. Recently, the soft lithographic graphene patterning technique has played an important role in the fabrication process of low-cost and flexible graphene-based electronics. In our recent study, we reported that a graphene sheet grown via chemical vapor deposition (CVD) was patterned and transferred to the target surface by surface energy modification using hydrophilic molecules on the surface of a polydimethylsiloxane (PDMS) stamp [4]. However, although a general route for the functionalization of graphene and graphene derivatives with inorganic materials has been reported, additional functionalization processes such as specific surface treatments, a doping process, or combining a process with inorganic materials is required. In addition, there are no reports of nanoscale sheets comprised of graphene oxide (GO) or GO composites suitable for mass fabrication or a solution-based inkjet printing process to pattern and transfer them to arbitrary substrates. In this study, we developed a soft lithographic patterning method for graphene-based flexible devices such as chemical sensors or biocompatible transistors. Here, the surface of an elastomeric stamp was coated with hydrophilic dimethylsulfoxide (DMSO) molecules to increase the surface energy. Graphene-based sheets comprised of a graphene-nanoparticle composite were removed from the initial substrate by contact of the PDMS stamp-coated DMSO and subsequently transferred to a target surface. In addition, we also successfully demonstrated a flexible graphene-based device on a biocompatible silk fibroin substrate, which is not suitable for conventional CMOS processes. Significantly, this graphene patterning method should provide considerable flexibility and could prove to be a key technology for building nanomechanical or bio-related systems based on graphene and its composites.
2. Experimental and results Figure 1 shows a schematic diagram of the fabrication of carbon material-based devices using the soft lithographic patterning method. First, GO and a GO-palladium nanoparticle (PdNP) composite were prepared from natural graphite powder by the modified Hummers method and a two-phase protocol, respectively [5]. GO or the GO-PdNP composite sheet on the solid substrate was obtained using a spin-coating deposition technique. In this coating process, the sheet thickness was controlled by adjusting the rotational speed of the coater or the number of deposition layers. The GO or GO-PdNP composite was spin-coated on a hydrophilic substrate at a slow rotation rate (1000 rpm) for 30 s, and this coating process was repeated three to five times for the graphene-based layer with a thickness of 2
M W Jung et al
Nanotechnology 25 (2014) 285302
Figure 1. A schematic diagram of the fabrication of the graphene-based device using the soft lithographic patterning method. (a) Spin coating of GO or GO-PdNPs on a SiO2/Si substrate. (b) Hydrazine treatment at 80 °C for 12 h for the reduction of GO. (c) The modification of the surface energy of PDMS by coating the pre-designed surface of an elastomeric stamp with DMSO. (d) The pattern fabrication of the rGO or rGO-PdNP by removing a specific area of the rGO or rGO-PdNP layer. (e) Transferring rGO or rGO-PdNP on the PDMS stamp to the target substrate. (f) Fabrication of metallic electrodes for a flexible graphene-based device.
Figure 2. (a) SEM image and Raman spectrum of the rGO patterns (blue) and SiO2 (red) surface after removing the specific rGO area from the initial substrate using a PDMS stamp. (b) Optical image of the boundary of the rGO patterns and Raman image showing the intensity of the G-band. (c) rGO patterns on the initial substrate (left) and target substrate (right) by transferring, using a PDMS stamp. (d) SEM images of rGO-based electrodes obtained by repeating the transfer method. (e) Optical image of the rGO patterns on the PET substrate.
fabrication of graphene-based electrodes on general electronic substrates. The GO thin film attached to the PDMS stamp was transferred to a target substrate, and an SEM image of the transferred rGO-based electrodes on the SiO2 surface is shown in the left image of figure 2(c). High-resolution rGO patterns were made using this positive patterning method on a SiO2 substrate by removing the specific rGO area in
contact with the elastomeric stamp. The rGO pieces on the contact area were removed completely since the interfacial energy between individual rGO pieces is greater than that between the rGO pieces and SiO2. In addition to the highresolution rGO patterns on the initial substrate, we transferred the rGO patterns to the target substrate using a DMSO-coated stamp by applying a process similar to that described in a previous work [4]. Cross rGO-based line 3
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Nanotechnology 25 (2014) 285302
Figure 3. Soft lithographic patterning of rGO-PdNP composites for application in chemical sensors. (a) TEM images of reduced graphene oxide sheets decorated with PdNPs. (b) SEM images of patterns based on the rGO-PdNP composite on a SiO2/Si substrate. (c) rGO-PdNPbased gas sensor. (d) Relative resistance response (ΔR/R) as a function of the injection time of hydrogen gas.
electrodes were also demonstrated by depositing GO-based electrodes on top of the first rGO line pattern. An rGO layer was patterned by contact with a line-shaped PDMS stamp, and these rGO pieces on the stamp were then transferred onto the GO electrodes on the initial SiO2 substrate. As shown in figure 2(d), the first transferred rGO patterns were still stable after the second transfer process. Significantly, this GO patterning method allowed us to fabricate graphenebased electrodes on a flexible substrate such as polyethylene phthalate (PET) (figure 2(e)). This method may pave the way towards flexible and transparent advanced graphene devices due to the simplicity and efficiency of our patterning technique. In addition to the patterning method of GO consisting only of carbon materials, we also successfully demonstrated high-resolution patterns comprised of rGO and a metallic nanoparticle composite for devices with a specific functionality, such as chemical sensors [9–12]. In a previous report, graphene-metal nanowire hybrid structures were adopted for high-performance, transparent, and stretchable electrodes [13]. In this study, GO and a PdNP composite were first synthesized by attaching PdNPs on the surface of GO sheets (see supporting information, available at stacks.iop.org/ NANO/25/285302/mmedia). As shown in the transmission electron microscopy (TEM) image of the GO-PdNP composite (figure 3(a)), PdNPs were assembled uniformly on the GO surface. Here, the density of the PdNPs was about 0.038 nm−2, and the average size of the PdNPs was
approximately 5 nm. The GO-PdNP composite was dispersed well in de-ionized water for at least a few days, then the GOPdNP suspension (0.5 mg ml−1) was spin-coated on a SiO2 substrate. The thickness of the rGO-PdNPs composite film was controlled by adjusting the spin coating speed or the concentration of the GO solution. After the reduction of GOPdNP to rGO-PdNPs, the PDMS stamp coated with DMSO molecules was contacted with the rGO-PdNPs layer. Only the rGO-PdNP composite on the contact region was removed completely. In the SEM image of the rGO-PdNP patterns over a large scale area (figure 3(b)), the dark line with a 5 μm width corresponds to the electrodes based on the rGO-PdNP composite, and the bright region is the bare SiO2 surface. We also fabricated hydrogen sensors based on the rGOPdNP composite using our soft lithographic patterning method (figures 3(c) and (d)). When palladium attached on a GO surface is exposed to H2 gas, it reacts reversibly to form PdHx, and its resistance changes at room temperature [14]. Figure 3(d) shows the typical response transient of the rGOPdNP composite-based sensors when they were repeatedly exposed to the sample gas (1 vol. % H2 balanced by air) and reference gas (air). We observed very reproducible resistance changes (ΔR) of ∼1.5%. It should be noted that our rGOPdNP composite-based sensors did not show any degradation or hysteresis after repeated exposure. This result implies that our process allows us to generate robust graphene-based devices suitable for practical applications such as a H2 detector in H2-based fuel cells. 4
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Nanotechnology 25 (2014) 285302
Figure 4. Fabrication of graphene-based flexible devices on a silk substrate. (a) The soft lithographic patterning method for graphene devices
on a silk substrate. (b) SEM image of an rGO-PdNP composite on a silk film. (c) SEM images of uniform CVD-graphene patterns with a width of 5 μm and (d) after transferring the graphene-based devices on the silk film to the 3D surface. (e) Transfer curve of the graphene device transferred to the 3D glass cylinder. The inset exhibits output characteristics of the graphene device.
Silk fibroin is a particularly appealing biopolymer candidate for forming biocompatible electronic devices because it is optically transparent, mechanically robust, and flexible in a thin-film form. Additionally, it is manageable to functionalize the silk surface with chemical and biological materials [15]. In this study, we successfully fabricated graphene-based flexible devices on a silk fibroin substrate using our soft lithographic patterning method. First, the process of preparing the silk substrates started with materials derived from Bombyx mori cocoons, following procedures similar to those previously reported [15–19]. The cocoons were degummed by boiling with Marseille soap in order to remove sericin. Degummed silk was solubilized in the CaCl2/C2H5OH/H2O (1:2:8 molar ratio) solution at 80 °C for 3 h. The solution was then filtered through a miracloth and dialyzed to remove the dissolved salt over the course of 3 d. Silk films were made by casting fibroin solutions onto PDMS and drying in air for 12–24 h, depending on the thickness. When crystallized in air, the silk fibroin secondary structure kinetically favors silk I formation and ∼50 μm thick films. In a way similar to the patterning method of GO pieces, we used a PDMS stamp coated with DMSO to remove the graphene-based materials from an initial substrate and transfer them to a target substrate. When the PDMS stamp was contacted with the hydrophilic silk surface at 60 °C for 5 min, the graphene-based materials were transferred to the silk substrate. Metallic electrodes were then fabricated on top of the patterns of the graphene-based materials via shadow maskassisted e-beam evaporation. Due to the wettability of silk fibroin films by an aqueous solution, the silk films were flushed with saline to dissolve part of the film for the attachment of silk-based hybrid devices on three-dimensional (3D) target substrates (figure 4(a)). Figure 4(b) shows an SEM image of
the GO-PdNPs composite patterns over a large scale area on a silk fibroin film. In addition to patterning of the GO-PdNPs composite, we could also apply the same technique for the patterns of CVD-grown graphene, which is suitable for the electrodes of flexible devices because of its higher conductance than GO thin films. As shown in figure 4(c), CVD-graphene patterns with a width of 5 μm were transferred on the silk fibroin film. Since CVD-graphene electrodes have a clear boundary without the use of photolithography in a subsequent etching step, our soft lithography technique is suitable for the fabrication of graphene electrodes on biocompatible and flexible devices. Here, the surface free energies of the silk fibroin substrate, PDMS, and the DMSO-coated PDMS surface were calculated using the classical Owens-Wendt methodology for dispersive and polar surface energy components, using the contact angles for diiodomethane and water. The calculated results are summarized in figure S1 in the Supporting Information. As shown in figure S1, because the surface free energy of the silk fibroin film (about 42.61 mJ m−2) is between the surface free energies of PDMS and DMSO-coated PDMS, the surface energy modification using hydrophilic molecules makes it possible to remove graphene-based materials from the initial substrate and to transfer them to the silk substrate. The SEM image of the CVD-graphene electrodes (figure 4(c)) also confirms that the graphene electrodes were transferred without physical damage of the graphene layer. Figures 4(d) and (e) show the silk-based graphene device transferred to a 3D structure and the electrical properties of the graphene device after transfer, respectively. When rinsing the silk substrate with DI water on the target surface, the adhesive force between the silk and glass cylinder increased. When the silk-graphene hybrid device was subsequently dried 5
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Nanotechnology 25 (2014) 285302
References
under ambient conditions, the hybrid device attached on the glass surface was continuously placed on the 3D substrate. The transfer characteristics of the graphene device transferred to the 3D glass cylinder were measured, and they showed ambipolar behavior and Ohmic contact. The Dirac point appeared at +0.3 V, demonstrating that p-type graphene was formed, and the field effect mobility was about 311.37 cm2 V−1 s, which is similar to the results of previous reports [20]. These results show that graphene/silk-based devices could be attached to arbitrary target substrates without physical damage of the graphene channels occurring during the transfer process. Significantly, considering that a lack of high-throughput graphene-based materials patterning methods has delayed the commercial application of graphene devices, this process can provide a key solution for bio-related applications and advanced flexible circuits comprised of graphenebased materials.
[1] Geim A K and Novoselov K S 2007 The rise of graphene Nat. Mater. 6 183 [2] Novoselov K S, Geim A K, Morozov S V, Jiang D, Katsnelson M I, Grigorieva I V, Dubonos S V and Firsov A A 2005 Two-dimensional gas of massless dirac fermions in graphene Nature 438 197 [3] Park J U, Nam S W, Lee M S and Lieber C M 2012 Synthesis of monlithic graphene-graphite intergrated electronics Nat. Mater. 11 120 [4] Kim H S, Jung M W, Myung S, Jung D S, Lee S S, Kong K J, Lim J S, Lee J H, Park C Y and An K S 2013 Soft lithography of graphene sheets via surface energy modification J. Mater. Chem. C 1 1076 [5] Liu L, Liu J, Wang Y, Yan X and Sun D D 2011 Facile synthesis of monodispersed silver nanoparticles on graphene oxide sheets with enhanced antibacterial activity New J. Chem. 35 1418 [6] Zhu Y, Mural S, Cai W, Li X, Suk J W, Potts J R and Ruoff R S 2010 Graphene and graphene oxide: synthesis, properties, and applications Adv. Mater. 22 3906 [7] Compton O C and Nguyen S T 2010 Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials Small 6 711 [8] Huang X, Qi X, Boey F and Zhang H 2012 Graphene-based composites Chem. Soc. Rev. 41 666 [9] Lange U, Hirsch T, Mirsky V M and Wolfbeis O S 2011 Hydrogen sensor based on a graphene—palladium nanocomposite Electrochim. Acta 56 3707 [10] Sundaram R S, Gómez-Navarro C, Balasubramanian K, Burghard M and Kern K 2008 Electrochemical modification of graphene Adv. Mater. 20 3050 [11] Xi P, Chen F, Xie G, Ma C, Liu H, Shao C, Wang J, Xu Z, Xud X and Zeng Z 2012 Surfactant free RGO/Pd nanocomposites as highly active heterogeneous catalysts for the hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage Nanoscale 4 5597 [12] Wang Q, Cui X, Chen J, Zheng X, Liu C, Xue T, Wang H, Jin Z, Qiao L and Zheng W 2012 Well-dispersed palladium nanoparticles on graphene oxide as a non-enzymatic glucose sensor RSC advances 2 6245 [13] Lee M S et al 2013 High-performance, transparent, and stretchable electrodes using graphene-metal nanowire hybrid structures Nano Lett. 13 2814 [14] Christoper Love J, Daniel B W, Richard H, Michael L C, Kateri E P, George M W and Ralph G N 2003 Formation and structure of self-assembled monolayers of alkanethiolates on palladium J. Am. Chem. Soc. 125 2597 [15] Mannoor M S, Tao H, Clayton J D, Sengupta A, Kaplan D L, Naik R R, Verma N, Omenetto F G and McAlpine M C 2012 Graphene-based wireless bacteria detection on tooth enamel Nat. Commun. 3 1767 [16] Kim D-H et al 2010 Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics Nat. Mat. 9 511 [17] Perry H, Gopinath A, Kaplan D L, Negro L D and Omenetto F G 2008 Nano-and micropatterning of optically transparent, mechanically robust, biocompatible silk fibroin films Adv. Mater. 20 3070 [18] Tao H, Kaplan D L and Omenetto F G 2012 Silk materials—a road to sustainable high technology Adv. Mater. 24 2824 [19] Tang Y, Cao C, Ma X, Chen C and Zhu H 2006 Study on the preparation of collagen-modified silk fibroin films and their properties Biomed. Mater. 1 242 [20] Lee S K, Kim B J, Jang H, Yoon S C, Lee C, Hong B H, Rogers J A, Cho J H and Ahn J H 2011 Stretchable graphene transistors with printed dielectrics and gate electrodes Nano Lett. 11 4642
3. Conclusion In summary, we successfully developed a soft lithographic graphene patterning method for flexible graphene-based devices. In this study, we utilized surface energy modification using hydrophilic coating molecules to transfer graphenebased materials, including graphene oxide or graphenemetallic NP composites, to a target substrate. As a proof of concept, a rGO-PdNP composite was patterned and transferred to a target substrate, and hydrogen sensors based on the rGO-PdNP composite were successfully demonstrated utilizing our graphene patterning technique. In addition, using our patterning technique, which is comparable with the conventional CMOS processes, graphene-based electrodes were fabricated on a silk fibroin substrate. These results confirm that graphene-based devices on a flexible substrate were also successfully transferred to a target surface without the loss of the electronic properties of the flexible graphene device on a silk fibroin substrate or the loss of physical properties during the transfer processes. The ease of fabrication and biocompatibility, along with the excellent physical properties of graphene-based materials, make our soft lithographic patterning method of graphene materials an ideal candidate for advanced flexible devices or bio-related applications.
Acknowledgments This research was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2012K001301) and by a grant (20110031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. 6
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My IOPscience
Fabrication of graphene-based flexible devices utilizing a soft lithographic patterning method
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 285302 (http://iopscience.iop.org/0957-4484/25/28/285302) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 130.133.8.114 This content was downloaded on 24/04/2017 at 12:24 Please note that terms and conditions apply.
You may also be interested in: Reduced graphene oxide nanoshells for flexible and stretchable conductors Wen-Shuai Jiang, Zhi-Bo Liu, Wei Xin et al. Defect-free functionalized graphene sensor for formaldehyde detection Xiaohui Tang, Nathalie Mager, Beatrice Vanhorenbeke et al. Enhancing gas sensing properties of graphene by using a nanoporous substrate Cheol-Soo Yang, Ather Mahmood, Bongseock Kim et al. Engineering electrical properties of graphene: chemical approaches Yong-Jin Kim, Yuna Kim, Konstantin Novoselov et al. Microorganism mediated synthesis of reduced graphene oxide films Y Tanizawa, Y Okamoto, K Tsuzuki et al. Programmable high crystallinity carbon patterns Xuewen Wang, Hong Wang, Yang Gu et al. Ultrafast and sensitive room temperature NH3 gas sensors based on chemically reduced graphene oxide Nantao Hu, Zhi Yang, Yanyan Wang et al. Sodium deoxycholate functionalized graphene and its composites with polyvinyl alcohol Lanwei Wang, Ruijuan Liao, Zhenghai Tang et al. Fabrication of highly oriented reduced graphene oxide microbelts array for massive production of sensitive ammonia gas sensors Jia Zhang, Rongfu Zhang, Xiaona Wang et al.
Nanotechnology Nanotechnology 25 (2014) 285302 (6pp)
doi:10.1088/0957-4484/25/28/285302
Fabrication of graphene-based flexible devices utilizing a soft lithographic patterning method Min Wook Jung1,2,4, Sung Myung1,4, Ki Woong Kim1, Wooseok Song1, You-Young Jo3, Sun Suk Lee1, Jongsun Lim1, Chong-Yun Park2 and Ki-Seok An1 1
Thin Film Materials Research Group, Korea Research Institute of Chemical Technology, Daejeon 305-543, Republic of Korea 2 Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea 3 National Academy of Agricultural Science, Rural Development Administration, Suwon 441-707, Republic of Korea E-mail: [email protected] Received 6 January 2014, revised 8 April 2014 Accepted for publication 20 May 2014 Published 27 June 2014 Abstract
There has been considerable interest in soft lithographic patterning processing of large scale graphene sheets due to the low cost and simplicity of the patterning process along with the exceptional electrical or physical properties of graphene. These properties include an extremely high carrier mobility and excellent mechanical strength. Recently, a study has reported that single layer graphene grown via chemical vapor deposition (CVD) was patterned and transferred to a target surface by controlling the surface energy of the polydimethylsiloxane (PDMS) stamp. However, applications are limited because of the challenge of CVD-graphene functionalization for devices such as chemical or bio-sensors. In addition, graphene-based layers patterned with a micron scale width on the surface of biocompatible silk fibroin thin films, which are not suitable for conventional CMOS processes such as the patterning or etching of substrates, have yet to be reported. Herein, we developed a soft lithographic patterning process via surface energy modification for advanced graphene-based flexible devices such as transistors or chemical sensors. Using this approach, the surface of a relief-patterned elastomeric stamp was functionalized with hydrophilic dimethylsulfoxide molecules to enhance the surface energy of the stamp and to remove the graphene-based layer from the initial substrate and transfer it to a target surface. As a proof of concept using this soft lithographic patterning technique, we demonstrated a simple and efficient chemical sensor consisting of reduced graphene oxide and a metallic nanoparticle composite. A flexible graphene-based device on a biocompatible silk fibroin substrate, which is attachable to an arbitrary target surface, was also successfully fabricated. Briefly, a soft lithographic patterning process via surface energy modification was developed for advanced graphene-based flexible devices such as transistors or chemical sensors and attachable devices on a biocompatible silk fibroin substrate. Significantly, this soft lithographic patterning technique enables us to demonstrate a simple and efficient chemical sensor based on reduced graphene oxide (rGO), a metallic nanoparticle composite, and an attachable graphene-based device on a silk fibroin thin film. S Online supplementary data available from stacks.iop.org/NANO/25/285302/mmedia Keywords: soft lithography, graphene, flexible devices (Some figures may appear in colour only in the online journal) 4
These authors contributed equally.
0957-4484/14/285302+06$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
M W Jung et al
Nanotechnology 25 (2014) 285302
∼30 nm. Hydrazine is the most common and effective reducing agent used to convert GO to reduced GO (rGO). As a result, the GO or GO-PdNP composite on the substrate was reduced overnight by hydrazine vapor at 80 °C, as described in previous studies [6–8]. The rGO and rGOPdNP composite have a black-colored and hydrophobic surface, and the rGO has a smaller surface energy than GO before reduction due to the hydrophobicity of the rGO surface. We optimized the surface energy of rGO and the rGO-PdNP composite as well as the adhesion between graphene-based materials and the initial substrate by the reduction of GO. In figure 1(c), hydrophilic molecules such as DMSO were used to enhance the adhesive force between the rGO (or rGO-PdNP) layer and the surface of the elastomeric mold. DMSO diluted in deionized (DI) water (DMSO:DI water = 1:2 (v:v)) was vaporized and coated on the PDMS surface at a level 5 cm above the diluted DMSO-water solution at 200 °C for 1 min. for the uniform deposition on the PDMS surface. When the PDMS mold was contacted with the layer of the graphene-based materials such as rGO or the rGO-PdNP composite for a few minutes, the graphene-based material was removed from the initial substrate and transferred to the PDMS stamp, which has a higher surface energy than the SiO2 substrate. When PDMS with the graphene-based materials contacted the surface of the target substrate for 2 mins. at 70 °C, the surface energy of the PDMS was restored. The graphene-based materials were subsequently detached from the PDMS surface (figure 1(e)). Finally, the source and drain electrodes were fabricated on the top of the graphenebased material patterns via e-beam evaporation of metals through a shadow mask. A scanning electron microscopy (SEM) image and the Raman spectrum of the rGO patterns obtained using the soft lithographic patterning method are shown in figure 2(a). As shown in the SEM image, we could obtain uniform rGO line electrodes with a high resolution (∼8 μm) width compared to a previous study, in which it was reported that rGO pieces were patterned without using any assistive compounds [1]. Here, the dark area in the SEM image of the rGO patterns represents the remaining rGO patterns after the removal of the rGO pieces. The bright area corresponds to the exposed SiO2 surface. The G-band in the Raman spectrum (right image of figure 2(a)) is located at 1580 cm−1, which originates from a first order Raman scattering process and corresponds to the in-plane vibration of sp2-bonded carbon atoms. The Raman peak at 1350 cm−1 is associated with defects in graphene, and it is usually referred to as the disorder-induced D-peak. The two-dimensional peak at 2680 cm−1 originates from a two phonon double resonance Raman process and is approximately twice the frequency of the D-peak. The Raman intensity map of the G-band of the rGO patterns is shown in figure 2(b). The differences between the rGO patterns and the bare SiO2 surface are obvious. It can be clearly seen that the bright area in the left optical microscopy image corresponds to rGO patterns remaining on the SiO2 substrate. These results show that our method can be readily utilized for the
1. Introduction In the past decade, various devices based on graphene have been developed due to its excellent electrical and physical characteristics, such as a high carrier mobility, thermal conductivity, mechanical flexibility, and optical transparency [1, 2]. However, a prerequisite for the development of graphene-based electronic devices is a reliable patterning technique [3]. Recently, the soft lithographic graphene patterning technique has played an important role in the fabrication process of low-cost and flexible graphene-based electronics. In our recent study, we reported that a graphene sheet grown via chemical vapor deposition (CVD) was patterned and transferred to the target surface by surface energy modification using hydrophilic molecules on the surface of a polydimethylsiloxane (PDMS) stamp [4]. However, although a general route for the functionalization of graphene and graphene derivatives with inorganic materials has been reported, additional functionalization processes such as specific surface treatments, a doping process, or combining a process with inorganic materials is required. In addition, there are no reports of nanoscale sheets comprised of graphene oxide (GO) or GO composites suitable for mass fabrication or a solution-based inkjet printing process to pattern and transfer them to arbitrary substrates. In this study, we developed a soft lithographic patterning method for graphene-based flexible devices such as chemical sensors or biocompatible transistors. Here, the surface of an elastomeric stamp was coated with hydrophilic dimethylsulfoxide (DMSO) molecules to increase the surface energy. Graphene-based sheets comprised of a graphene-nanoparticle composite were removed from the initial substrate by contact of the PDMS stamp-coated DMSO and subsequently transferred to a target surface. In addition, we also successfully demonstrated a flexible graphene-based device on a biocompatible silk fibroin substrate, which is not suitable for conventional CMOS processes. Significantly, this graphene patterning method should provide considerable flexibility and could prove to be a key technology for building nanomechanical or bio-related systems based on graphene and its composites.
2. Experimental and results Figure 1 shows a schematic diagram of the fabrication of carbon material-based devices using the soft lithographic patterning method. First, GO and a GO-palladium nanoparticle (PdNP) composite were prepared from natural graphite powder by the modified Hummers method and a two-phase protocol, respectively [5]. GO or the GO-PdNP composite sheet on the solid substrate was obtained using a spin-coating deposition technique. In this coating process, the sheet thickness was controlled by adjusting the rotational speed of the coater or the number of deposition layers. The GO or GO-PdNP composite was spin-coated on a hydrophilic substrate at a slow rotation rate (1000 rpm) for 30 s, and this coating process was repeated three to five times for the graphene-based layer with a thickness of 2
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Nanotechnology 25 (2014) 285302
Figure 1. A schematic diagram of the fabrication of the graphene-based device using the soft lithographic patterning method. (a) Spin coating of GO or GO-PdNPs on a SiO2/Si substrate. (b) Hydrazine treatment at 80 °C for 12 h for the reduction of GO. (c) The modification of the surface energy of PDMS by coating the pre-designed surface of an elastomeric stamp with DMSO. (d) The pattern fabrication of the rGO or rGO-PdNP by removing a specific area of the rGO or rGO-PdNP layer. (e) Transferring rGO or rGO-PdNP on the PDMS stamp to the target substrate. (f) Fabrication of metallic electrodes for a flexible graphene-based device.
Figure 2. (a) SEM image and Raman spectrum of the rGO patterns (blue) and SiO2 (red) surface after removing the specific rGO area from the initial substrate using a PDMS stamp. (b) Optical image of the boundary of the rGO patterns and Raman image showing the intensity of the G-band. (c) rGO patterns on the initial substrate (left) and target substrate (right) by transferring, using a PDMS stamp. (d) SEM images of rGO-based electrodes obtained by repeating the transfer method. (e) Optical image of the rGO patterns on the PET substrate.
fabrication of graphene-based electrodes on general electronic substrates. The GO thin film attached to the PDMS stamp was transferred to a target substrate, and an SEM image of the transferred rGO-based electrodes on the SiO2 surface is shown in the left image of figure 2(c). High-resolution rGO patterns were made using this positive patterning method on a SiO2 substrate by removing the specific rGO area in
contact with the elastomeric stamp. The rGO pieces on the contact area were removed completely since the interfacial energy between individual rGO pieces is greater than that between the rGO pieces and SiO2. In addition to the highresolution rGO patterns on the initial substrate, we transferred the rGO patterns to the target substrate using a DMSO-coated stamp by applying a process similar to that described in a previous work [4]. Cross rGO-based line 3
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Figure 3. Soft lithographic patterning of rGO-PdNP composites for application in chemical sensors. (a) TEM images of reduced graphene oxide sheets decorated with PdNPs. (b) SEM images of patterns based on the rGO-PdNP composite on a SiO2/Si substrate. (c) rGO-PdNPbased gas sensor. (d) Relative resistance response (ΔR/R) as a function of the injection time of hydrogen gas.
electrodes were also demonstrated by depositing GO-based electrodes on top of the first rGO line pattern. An rGO layer was patterned by contact with a line-shaped PDMS stamp, and these rGO pieces on the stamp were then transferred onto the GO electrodes on the initial SiO2 substrate. As shown in figure 2(d), the first transferred rGO patterns were still stable after the second transfer process. Significantly, this GO patterning method allowed us to fabricate graphenebased electrodes on a flexible substrate such as polyethylene phthalate (PET) (figure 2(e)). This method may pave the way towards flexible and transparent advanced graphene devices due to the simplicity and efficiency of our patterning technique. In addition to the patterning method of GO consisting only of carbon materials, we also successfully demonstrated high-resolution patterns comprised of rGO and a metallic nanoparticle composite for devices with a specific functionality, such as chemical sensors [9–12]. In a previous report, graphene-metal nanowire hybrid structures were adopted for high-performance, transparent, and stretchable electrodes [13]. In this study, GO and a PdNP composite were first synthesized by attaching PdNPs on the surface of GO sheets (see supporting information, available at stacks.iop.org/ NANO/25/285302/mmedia). As shown in the transmission electron microscopy (TEM) image of the GO-PdNP composite (figure 3(a)), PdNPs were assembled uniformly on the GO surface. Here, the density of the PdNPs was about 0.038 nm−2, and the average size of the PdNPs was
approximately 5 nm. The GO-PdNP composite was dispersed well in de-ionized water for at least a few days, then the GOPdNP suspension (0.5 mg ml−1) was spin-coated on a SiO2 substrate. The thickness of the rGO-PdNPs composite film was controlled by adjusting the spin coating speed or the concentration of the GO solution. After the reduction of GOPdNP to rGO-PdNPs, the PDMS stamp coated with DMSO molecules was contacted with the rGO-PdNPs layer. Only the rGO-PdNP composite on the contact region was removed completely. In the SEM image of the rGO-PdNP patterns over a large scale area (figure 3(b)), the dark line with a 5 μm width corresponds to the electrodes based on the rGO-PdNP composite, and the bright region is the bare SiO2 surface. We also fabricated hydrogen sensors based on the rGOPdNP composite using our soft lithographic patterning method (figures 3(c) and (d)). When palladium attached on a GO surface is exposed to H2 gas, it reacts reversibly to form PdHx, and its resistance changes at room temperature [14]. Figure 3(d) shows the typical response transient of the rGOPdNP composite-based sensors when they were repeatedly exposed to the sample gas (1 vol. % H2 balanced by air) and reference gas (air). We observed very reproducible resistance changes (ΔR) of ∼1.5%. It should be noted that our rGOPdNP composite-based sensors did not show any degradation or hysteresis after repeated exposure. This result implies that our process allows us to generate robust graphene-based devices suitable for practical applications such as a H2 detector in H2-based fuel cells. 4
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Figure 4. Fabrication of graphene-based flexible devices on a silk substrate. (a) The soft lithographic patterning method for graphene devices
on a silk substrate. (b) SEM image of an rGO-PdNP composite on a silk film. (c) SEM images of uniform CVD-graphene patterns with a width of 5 μm and (d) after transferring the graphene-based devices on the silk film to the 3D surface. (e) Transfer curve of the graphene device transferred to the 3D glass cylinder. The inset exhibits output characteristics of the graphene device.
Silk fibroin is a particularly appealing biopolymer candidate for forming biocompatible electronic devices because it is optically transparent, mechanically robust, and flexible in a thin-film form. Additionally, it is manageable to functionalize the silk surface with chemical and biological materials [15]. In this study, we successfully fabricated graphene-based flexible devices on a silk fibroin substrate using our soft lithographic patterning method. First, the process of preparing the silk substrates started with materials derived from Bombyx mori cocoons, following procedures similar to those previously reported [15–19]. The cocoons were degummed by boiling with Marseille soap in order to remove sericin. Degummed silk was solubilized in the CaCl2/C2H5OH/H2O (1:2:8 molar ratio) solution at 80 °C for 3 h. The solution was then filtered through a miracloth and dialyzed to remove the dissolved salt over the course of 3 d. Silk films were made by casting fibroin solutions onto PDMS and drying in air for 12–24 h, depending on the thickness. When crystallized in air, the silk fibroin secondary structure kinetically favors silk I formation and ∼50 μm thick films. In a way similar to the patterning method of GO pieces, we used a PDMS stamp coated with DMSO to remove the graphene-based materials from an initial substrate and transfer them to a target substrate. When the PDMS stamp was contacted with the hydrophilic silk surface at 60 °C for 5 min, the graphene-based materials were transferred to the silk substrate. Metallic electrodes were then fabricated on top of the patterns of the graphene-based materials via shadow maskassisted e-beam evaporation. Due to the wettability of silk fibroin films by an aqueous solution, the silk films were flushed with saline to dissolve part of the film for the attachment of silk-based hybrid devices on three-dimensional (3D) target substrates (figure 4(a)). Figure 4(b) shows an SEM image of
the GO-PdNPs composite patterns over a large scale area on a silk fibroin film. In addition to patterning of the GO-PdNPs composite, we could also apply the same technique for the patterns of CVD-grown graphene, which is suitable for the electrodes of flexible devices because of its higher conductance than GO thin films. As shown in figure 4(c), CVD-graphene patterns with a width of 5 μm were transferred on the silk fibroin film. Since CVD-graphene electrodes have a clear boundary without the use of photolithography in a subsequent etching step, our soft lithography technique is suitable for the fabrication of graphene electrodes on biocompatible and flexible devices. Here, the surface free energies of the silk fibroin substrate, PDMS, and the DMSO-coated PDMS surface were calculated using the classical Owens-Wendt methodology for dispersive and polar surface energy components, using the contact angles for diiodomethane and water. The calculated results are summarized in figure S1 in the Supporting Information. As shown in figure S1, because the surface free energy of the silk fibroin film (about 42.61 mJ m−2) is between the surface free energies of PDMS and DMSO-coated PDMS, the surface energy modification using hydrophilic molecules makes it possible to remove graphene-based materials from the initial substrate and to transfer them to the silk substrate. The SEM image of the CVD-graphene electrodes (figure 4(c)) also confirms that the graphene electrodes were transferred without physical damage of the graphene layer. Figures 4(d) and (e) show the silk-based graphene device transferred to a 3D structure and the electrical properties of the graphene device after transfer, respectively. When rinsing the silk substrate with DI water on the target surface, the adhesive force between the silk and glass cylinder increased. When the silk-graphene hybrid device was subsequently dried 5
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References
under ambient conditions, the hybrid device attached on the glass surface was continuously placed on the 3D substrate. The transfer characteristics of the graphene device transferred to the 3D glass cylinder were measured, and they showed ambipolar behavior and Ohmic contact. The Dirac point appeared at +0.3 V, demonstrating that p-type graphene was formed, and the field effect mobility was about 311.37 cm2 V−1 s, which is similar to the results of previous reports [20]. These results show that graphene/silk-based devices could be attached to arbitrary target substrates without physical damage of the graphene channels occurring during the transfer process. Significantly, considering that a lack of high-throughput graphene-based materials patterning methods has delayed the commercial application of graphene devices, this process can provide a key solution for bio-related applications and advanced flexible circuits comprised of graphenebased materials.
[1] Geim A K and Novoselov K S 2007 The rise of graphene Nat. Mater. 6 183 [2] Novoselov K S, Geim A K, Morozov S V, Jiang D, Katsnelson M I, Grigorieva I V, Dubonos S V and Firsov A A 2005 Two-dimensional gas of massless dirac fermions in graphene Nature 438 197 [3] Park J U, Nam S W, Lee M S and Lieber C M 2012 Synthesis of monlithic graphene-graphite intergrated electronics Nat. Mater. 11 120 [4] Kim H S, Jung M W, Myung S, Jung D S, Lee S S, Kong K J, Lim J S, Lee J H, Park C Y and An K S 2013 Soft lithography of graphene sheets via surface energy modification J. Mater. Chem. C 1 1076 [5] Liu L, Liu J, Wang Y, Yan X and Sun D D 2011 Facile synthesis of monodispersed silver nanoparticles on graphene oxide sheets with enhanced antibacterial activity New J. Chem. 35 1418 [6] Zhu Y, Mural S, Cai W, Li X, Suk J W, Potts J R and Ruoff R S 2010 Graphene and graphene oxide: synthesis, properties, and applications Adv. Mater. 22 3906 [7] Compton O C and Nguyen S T 2010 Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials Small 6 711 [8] Huang X, Qi X, Boey F and Zhang H 2012 Graphene-based composites Chem. Soc. Rev. 41 666 [9] Lange U, Hirsch T, Mirsky V M and Wolfbeis O S 2011 Hydrogen sensor based on a graphene—palladium nanocomposite Electrochim. Acta 56 3707 [10] Sundaram R S, Gómez-Navarro C, Balasubramanian K, Burghard M and Kern K 2008 Electrochemical modification of graphene Adv. Mater. 20 3050 [11] Xi P, Chen F, Xie G, Ma C, Liu H, Shao C, Wang J, Xu Z, Xud X and Zeng Z 2012 Surfactant free RGO/Pd nanocomposites as highly active heterogeneous catalysts for the hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage Nanoscale 4 5597 [12] Wang Q, Cui X, Chen J, Zheng X, Liu C, Xue T, Wang H, Jin Z, Qiao L and Zheng W 2012 Well-dispersed palladium nanoparticles on graphene oxide as a non-enzymatic glucose sensor RSC advances 2 6245 [13] Lee M S et al 2013 High-performance, transparent, and stretchable electrodes using graphene-metal nanowire hybrid structures Nano Lett. 13 2814 [14] Christoper Love J, Daniel B W, Richard H, Michael L C, Kateri E P, George M W and Ralph G N 2003 Formation and structure of self-assembled monolayers of alkanethiolates on palladium J. Am. Chem. Soc. 125 2597 [15] Mannoor M S, Tao H, Clayton J D, Sengupta A, Kaplan D L, Naik R R, Verma N, Omenetto F G and McAlpine M C 2012 Graphene-based wireless bacteria detection on tooth enamel Nat. Commun. 3 1767 [16] Kim D-H et al 2010 Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics Nat. Mat. 9 511 [17] Perry H, Gopinath A, Kaplan D L, Negro L D and Omenetto F G 2008 Nano-and micropatterning of optically transparent, mechanically robust, biocompatible silk fibroin films Adv. Mater. 20 3070 [18] Tao H, Kaplan D L and Omenetto F G 2012 Silk materials—a road to sustainable high technology Adv. Mater. 24 2824 [19] Tang Y, Cao C, Ma X, Chen C and Zhu H 2006 Study on the preparation of collagen-modified silk fibroin films and their properties Biomed. Mater. 1 242 [20] Lee S K, Kim B J, Jang H, Yoon S C, Lee C, Hong B H, Rogers J A, Cho J H and Ahn J H 2011 Stretchable graphene transistors with printed dielectrics and gate electrodes Nano Lett. 11 4642
3. Conclusion In summary, we successfully developed a soft lithographic graphene patterning method for flexible graphene-based devices. In this study, we utilized surface energy modification using hydrophilic coating molecules to transfer graphenebased materials, including graphene oxide or graphenemetallic NP composites, to a target substrate. As a proof of concept, a rGO-PdNP composite was patterned and transferred to a target substrate, and hydrogen sensors based on the rGO-PdNP composite were successfully demonstrated utilizing our graphene patterning technique. In addition, using our patterning technique, which is comparable with the conventional CMOS processes, graphene-based electrodes were fabricated on a silk fibroin substrate. These results confirm that graphene-based devices on a flexible substrate were also successfully transferred to a target surface without the loss of the electronic properties of the flexible graphene device on a silk fibroin substrate or the loss of physical properties during the transfer processes. The ease of fabrication and biocompatibility, along with the excellent physical properties of graphene-based materials, make our soft lithographic patterning method of graphene materials an ideal candidate for advanced flexible devices or bio-related applications.
Acknowledgments This research was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2012K001301) and by a grant (20110031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. 6
