Article pubs.acs.org/JPCC
Laser-Assisted Simultaneous Patterning and Transferring of Graphene Joon-Suk Oh,† Sang-Hoon Kim,‡ Taeseon Hwang,† Hyuk-Yong Kwon,§ Tae Hee Lee,∥ Ah-Hyun Bae,∥ Hyouk Ryeol Choi,§ and Jae-Do Nam*,†,‡ †
Department of Polymer Science and Engineering, ‡Department of Energy Science, and §School of Mechanical Engineering, Sungkyunkwan University, Jangan-gu, Suwon, 440-746, South Korea ∥ Manufacturing Core Technology Team, Global Production Technology Center, Samsung Electronics Co. Ltd., Maetan-dong, Yeongtong-gu, Suwon, South Korea, 443-742 S Supporting Information *
ABSTRACT: The patterning of graphene has gained a great deal of attention for practical applications such as electrical devices and sensors. Here we introduce a facile, versatile, and direct patterning method for the fabrication of electrically conductive graphene patterns on a flexible plastic substrate based on the laser transmission welding technique. One of the distinctive features of the developed technique is that both the patterning and transferring processes take place simultaneously with a simple laser treatment. Selective absorption of laser and localized melting by laser-induced heat were exploited to achieve a completely isolated pattern, occurring at the interface between a laser-absorbent graphene film and a laser-transparent plastic substrate. Graphene oxide (G-O) film was treated in the same way, resulting in a reduced G-O (RG-O) pattern. In this case, deoxygenation of functional groups in G-O arose together with the patterning and transferring. We found that the intensity and the scanning rate of laser irradiation considerably affected the size and chemical structures of the pattern. Scanning electron microscopy and Raman spectroscopy were used to measure the changes of the laser-treated G-O patterns.
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INTRODUCTION Graphene, a single layer of graphite, has attracted a great deal of attention due to its fascinating electronic, optical, thermal, and mechanical properties.1,2 It has been extensively studied in various fields for its potential applications toward transparent electrodes, field effect transistors, sensors, nanocomposites, and so on.3−7 Currently, there are several methods to obtain graphene including the mechanical cleavage of graphite, chemical vapor deposition (CVD) from hydrocarbon, and solution-based processes.8−12 Among them, a solution-phase chemical exfoliation of graphite has been often adopted to achieve a large quantity of graphene oxide (G-O) with ease. The G-O contains various oxidized functional groups on its basal plane and edges, which could be subsequently reduced to graphene.13 These functionalities render G-O hydrophilic and provide reactive sites for further modification. In addition, they create structural defects in G-O, making it insulating. Thus, a post-treatment such as chemical reduction or thermal annealing is needed to recover its electronic properties. Recently, the patterning of graphene has been of great interest for the fabrication of graphene-based electronic devices regarding practical applications including transparent electrodes, transistors, sensors, and so on.14−20 Conventional patterning methods such as photolithography and transfer printing have been adopted to create graphene patterns.14−18 © 2012 American Chemical Society
However, these techniques usually require numerous processes for the patterning of graphene sheets such as preparation of photomasks, prepatterned elastomeric stamps, UV exposure, developments, and so on. The latest reports introduced the direct laser patterning of G-O sheets, where the laser-focused region of the G-O layer was selectively converted to reduced graphene oxide (RG-O) in situ.19,20 These laser techniques showed great potential for high through-put and parallel processes with reliability and design flexibility at a low cost. However, the critical drawback of those techniques is that the laser-untreated G-O area remains in the same plane with the RG-O pattern region, whereby it could cause significant problems over time, because the G-O, known to be insulative, could be gradually converted to RG-O by heat or light. Thus, the residing G-O sheets should be removed or the RG-O pattern should be transferred to provide a complete pattern for real applications; however, nothing has been reported on this issue yet. To solve this problem, we employed the laser transmission welding (LTW) technique, which is a noncontact welding method for thermoplastics to join a laser-transparent part and a Received: September 21, 2012 Revised: December 5, 2012 Published: December 11, 2012 663
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Figure 1. Schematic of the laser-assisted selective transfer patterning of graphene and G-O film: (a) cross-section and (b) overall views.
Figure 2. Digital images of graphene patterns on PET film (zigzag patterns) (a), laser-treated graphene film (square patterns) located between PET films before (b) and after the detaching process (c), and a magnified graphene pattern (d). SEM image of the selected area in a graphene pattern (e, scale bar = 200 μm) and a tilted SEM image (f, scale bar = 40 μm).
laser-absorbent part.21,22 Our method enables the patterning and the transferring of graphene on a flexible substrate simultaneously through a single-step process. It is worth noting that this technique is applicable not only to graphene but also to G-O. We believe that the developed technique is a highly practical way to fabricate complete graphene patterns for potential applications.
the SK Corporation (Korea). The anodic aluminum oxide (AAO) membrane filters (Whatman, Inc., U.K.) were used for the vacuum filtration process. Preparation of Graphene and Graphene Oxide Film. Graphene and G-O suspension were prepared prior to film fabrication. For the preparation of graphene suspension, natural graphite, HNO3, and KMnO4 were mixed with a weight ratio of 1:1:2 in a porcelain dish.11 Then, the mixture was irradiated by using a microwave oven (X2-20MS, Whirlpool) at 700 W for 60 s to obtain expanded graphite (EG). The EG was exfoliated in 100 mL of NMP through sonication (VC-505, Sonics) for 1 h. Then, the suspension was centrifuged for 20 min at 3000 rpm (Combi-514R, Hanil) to quickly remove unexfoliated graphite, which gave a well-dispersed graphene suspension. For the preparation of the G-O suspension, a mixture of H2SO4 (360 mL) and H3PO4 (40 mL) was added to the graphite flakes
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EXPERIMENTAL SECTION Materials. Natural graphite flakes, nitric acid (68%, HNO3), sulfuric acid (95 - 97%, H2SO4), phosphoric acid (85%, H3PO4), hydrogen chloride (HCl), hydrogen peroxide (30%, H2O2), potassium permanganate (KMnO4), and N-methyl pyrollidone (NMP) were purchased from Sigma-Aldrich. Deionized water was used throughout the experiments. Poly(ethylene terephthalate) (PET) film was purchased from 664
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(10 g), and KMnO4 (40 g) was added to the mixture with gentle stirring.12 The mixture was then heated to 50 °C, and the oxidative reaction of graphite was carried out for 3 h. After the reaction, deionized water (2000 mL) with H2O2 (100 mL) was added to the mixture and stirred for 30 min to remove impurities. Afterward, the oxidized graphite flakes were collected by filtration, and the filtrate was washed several times with deionized water having 10% HCl. Finally, the oxidized graphite flakes were freeze-dried. The oxidized graphite (0.05 g) was added to deionized water (50 mL) and exfoliated by sonication for 4 h, followed by centrifugation of the suspension for 20 min at 3000 rpm to provide a stable graphene oxide suspension. Graphene and G-O films were prepared by vacuum filtration for each suspension obtained above through an AAO membrane filter (pore size 200 nm). The films were dried for 24 h at room temperature. Once the film on the AAO membrane was slowly immersed into water with the membrane side facing down, the graphene and G-O film were separated from the AAO membrane, where they began to float on the surface of the water. Subsequently, they were carefully transferred onto PET substrate. The transferred film was dried at 80 °C for 24 h. Laser-Assisted Selective Transfer Patterning (LASTP). A continuous wave laser diode with a wavelength of 976 nm was used (BrightLase Ultra-50, fiber core diameter 200 μm). The laser beam with an output power ranging from 500 to 1000 mW was focused on the graphene and G-O films placed between PET substrates and was scanned at a speed of 10 mm/ sec−1 with slight pressure. Following laser treatment, the upper PET substrate was detached from the bottom. Characterization. A scanning electron microscopy (SEM, Hitachi S2400) investigation was performed on the graphene pattern. X-ray photoelectron spectroscopy (XPS) (ESCA 2000, VG Microtech) was performed with a monochromatic Al Ka Xray source. The Raman spectra were obtained with RXN1 (Kaiser) using 633 nm Ar+ laser excitation. The sheet resistance of the G-O film following the laser treatment was measured by the four-point probe technique (Universal Probe, Jandel) with a source meter. The sample for the sheet resistance measurement was prepared through the repeated irradiation of the laser on G-O film (10 mm × 10 mm sample).
Figure 3. Digital image of a LG-O pattern on PET film (a). Raman spectra of G-O and LG-O (b). XPS wide scan (c) and C1s (d) spectra of G-O and LG-O.
and after the detaching process, respectively. In Figure 2b, we applied different intensities of the laser beam to discover the minimum laser power for completely transferred patterns (600 mW in line 1 and 2550 mW left and 500 mW right in line 3). When the laser beam was irradiated on the sample, a slight color change in the irradiated region was observed. We speculated that the refractivity of the PET of the site was altered in some degree following irradiation because the area of the PET had undergone both melting and solidifying processes. In Figure 2c, after the detachment of the sample, the graphene patterns are directly transferred to the PET substrate, whereas the laser-untreated area remains on the bottom substrate. However, the pattern applied with 550 mW of the laser beam is incompletely transferred and the other with 500 mW is not transferred. Because the amount of heat generated by laser absorption is proportional to the laser power, the output power of 600 mW must be the minimum level to obtain a complete pattern in this case. Figure 2d shows a completely patterned and transferred graphene on a PET substrate. Figure 2e displays an SEM image regarding the selected area of the pattern, where the line and the space widths are approximately 290 and 140 μm, respectively. Figure 2f is a tilted SEM image, in which we can clearly identify the graphene pattern and the PET substrate by their distinct features. In addition, Raman analysis was conducted on the pristine graphene and laser-treated graphene pattern, but no significant changes were observed, which implied that no structural transformation occurred by the laser treatment (Figure S1 in Supporting Information). Several laser techniques have been suggested to make graphene patterns, where the laser-focused area of G-O was selectively etched or reduced.19,20 However, they could only achieve an imperfectly etched G-O pattern or a selectively reduced G-O pattern surrounded by unreduced G-O sheets, which could be a potential risk in practical applications. Therefore, additional processes are required to obtain complete patterns; however, nothing has been reported on it yet. In
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RESULTS AND DISCUSSION Figure 1 illustrates the schematic of the fabrication process for a graphene pattern on thermoplastic substrate (herein PET was used) by the LASTP technique. Graphene (or G-O) film is placed between two substrates, where the upper one is transparent for the laser beam. When a laser is focused onto the sample with slight pressure ensuring close contact between the upper substrate and the graphene film, it passes through the transparent part and is absorbed onto the surface of the graphene. The energy of the absorbed beam is transformed into localized heat, which melts the upper plastic substrate by heat conduction. On cooling, the interface of the laser-irradiated region of the graphene and the substrate is strongly welded together. Subsequently, by simple detachment, the graphene pattern is transferred to the upper substrate, which gives rise to a graphene-patterned PET film. Figure 2 presents the results for the LASTP of graphene on PET film. Figure 2a shows a digital image of graphene patterns on a PET film with a zigzag design, where the patterns are clearly identified with the naked eye. Figure 2b,c displays the square design of laser-treated graphene patterns on PET before 665
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Figure 4. Graphs show the changes in the pattern width of LG-O at a constant scan rate (a) and at a constant output power (b). Corresponding SEM images (c,d, scale bar = 100 μm).
and 2950 cm−1 with weak intensity, where the 2D band is attributed to an out-of-plane vibration mode and the S3 band is due to the combination of D and G bands. It demonstrates that the 2D of LG-O is slightly increased due to the laser-induced graphitization of G-O.23 Figure 3c shows the XPS wide scan spectra of the G-O and LG-O samples. As can be seen, the O1s peak of the G-O substantially decreased compared with that of the LG-O, demonstrating that a substantial number of the oxygen functional groups were removed.23,25 The C1s spectra in Figure 3d compares the intensity of the C−C and C−O/CO binding energies of the G-O and LG-O. The considerable decrease in the content of C−O/CO following laser irradiation also supports the loss of oxygen groups.23,25 The electrical property of the LG-O was recovered, and the sheet resistance was measured to be ∼70 Ω square−1. From the results, we can expect that the electrical properties can be precisely modulated for various applications by several factors such as laser intensity, irradiation time, repetition of the irradiation, and so on. Because the absorbed laser beam is instantaneously transformed into local heat, the heat conduction between the interface can be explained by the conventional heat diffusion mechanism.26 The governing differential equation for diffusive heat conduction can be written in terms of temperature T and time t:
contrast, our technique could create complete graphene patterns on a flexible substrate in a single-step process.19,20 To our knowledge, this is the first report allowing the direct patterning and transferring of graphene to a substrate, which is a great advantage to fabricate electric devices in a practical manner. We also applied the LASTP to G-O film under similar conditions. Figure 3a presents a digital image of a laser-treated G-O pattern (LG-O) on PET. Similarly, the LG-O pattern was also easily transferred to PET as it became electrically conductive, whereas the untreated one is insulating. This indicates the reduction of G-O to RG-O, which is derived from the removal of the oxygen functional groups residing in the GO and the reconstruction of the G-O by the heat generated through laser absorption. This result is consistent with the report demonstrating that the insulating G-O is converted to be electrically conductive upon thermal treatment.19,23 The Raman spectra in Figure 3b confirm the structural transformation of GO by laser irradiation. The D bands around 1350 cm−1 and the G bands around 1580 cm−1 of G-O and LG-O can be seen. The prominent D band is related to the structural imperfection caused by the incorporation of the oxygen-containing functional groups during the oxidation of graphite and the G band to the recovery of the hexagonal structure of carbon atoms from defected structures.23,24 Following laser irradiation, the intensity ratio of the D and G bands (ID/IG) regarding G-O is significantly reduced from 1.06 to 0.74, indicating that the crystal defects were substantially reduced and graphitization took place. The 2D and S3 bands are observed around 2700
ρCp 666
∂T = ∇·(k∇T ) + q ∂t
(1)
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Figure 6. Intensity ratios of D and G bands (ID/IG) for the LG-O patterns at a constant scan rate with different laser intensities (a) and at a constant laser power with different scanning rates (b).
Figure 5. Raman spectra of LG-O patterns at a constant scan rate with different laser intensities (a) and at a constant laser power with different scan rates (b).
where ρ, Cp, k, and q represent the mass density, specific heat, thermal conductivity, and heat generation (W/m3), respectively. Assuming that the Lambert−Beer law is valid, the heat generation by the absorption of laser irradiation on a material is as follows:27
q(z) = αI0e−αz
speed increases at a constant laser power (700 mw). Because the heat produced by laser absorption is proportional to the laser power and interaction time (opposite to scanning rate), wider patterns were obtained at a higher power or at a slower scanning speed. Figure 4c,d shows the corresponding SEM images of the LG-O patterns, where the changes of the pattern width can be clearly seen. To examine the degree of the structural conversion for G-O, the above LG-O patterns were measured by Raman spectroscopy. The D band around 1350 cm−1 gradually decreases while the G band around 1580 cm−1 gradually increases over the laser intensity (Figure 5a) and interaction time (Figure 5b). The corresponding ID/IG values are shown in Figure 6, where the value constantly diminishes as the laser power and treatment time increase. It demonstrates the gradual elimination of defects and continuous graphitization of LG-O. These results imply that the degree of the reduction and the pattern size can be finely regulated with the laser intensity and scanning rate, which can be a useful option to control the pattern design and properties.
(2) 2
where the I0 is the laser intensity (W/m ) at the surface, z is the distance from the surface perpendicularly, and α is the absorption coefficient. According to the above equations, it is anticipated that the heat generation can be finely controlled by adjusting several factors such as absorption properties, laser power, laser diameter, and interaction time. A recent report has shown a simulation result in which the temperature on the surface of G-O film by laser irradiation gradually increases as the absorption coefficient increases.20 Figure 4 demonstrates the effect of the laser intensity and the scanning rate on the pattern width. In Figure 4a, we can clearly observe the increase in the pattern width of LG-O as the laser intensity increases at a constant scan speed (30 mm/sec). In Figure 4b, the pattern width gradually decreases as the scanning 667
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CONCLUSIONS By employing the laser welding technique, we have developed a versatile technique enabling the direct patterning and transferring of graphene and G-O on flexible plastic substrates. The laser-induced localized heat on graphene, and G-O films result in the localized melting of PET by heat conduction, followed by robust welding between the interface on cooling. Above the minimum laser intensity, the laser-directed pattern was completely transferred to the upper substrate by a simple detachment process. Additionally, we found that the heat generated by the absorption of laser converted the structures of G-O into the deoxygenated form, RG-O, which made the LG-O pattern electrically conductive. Furthermore, by varying the laser intensity and scanning rate, we could control the pattern size and the reduction level of LG-O. Compared with the existing patterning methods, our technique appears to be very practical and feasible because patterning and transferring take place simulatneously. We believe that this is a promising technique with great potential for application in various fields. By optimizing parameters such as laser beam diameters, powers, and scanning rates, it is strongly anticipated that this technique could be employed in the fabrication processes of electrical devices and sensors based on graphene materials.
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ASSOCIATED CONTENT
S Supporting Information *
Raman spectra of the pristine graphene and laser-treated graphene. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +82-31-299-4062. Fax: +82-31299-4069. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Samsung Electronics Co., Ltd. This work was also supported by the WCU (World Class University) program (R31-2008-10029) and the research grant (2010-0028939) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology. We also appreciate the project and equipment support from Gyeonggi Province through the GRRC program in Sungkyunkwan University.
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