Simple and Reliable Lift-Off Patterning Approach for Graphene and

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Simple and Reliable Lift-Off Patterning Approach for Graphene and Graphene-Ag Nanowire Hybrid Films Tran Nam Trung, Dong-Ok Kim, Jin-Hyung Lee, Van-Duong Dao, Ho-Suk Choi, and Eui-Tae Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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ACS Applied Materials & Interfaces

Simple and Reliable Lift-Off Patterning Approach for Graphene and Graphene-Ag Nanowire Hybrid Films

Tran Nam Trung,a Dong-Ok Kim,a Jin-Hyung Lee,a Van-Duong Dao,b Ho-Suk Choi,b and EuiTae Kima* a

Department of Materials Science & Engineering, Chungnam National University, Daejeon 305764, Korea b

Department of Chemical Engineering & Applied Chemistry, Chungnam National University, Daejeon, 305-764, Korea

ABSTRACT We present a simple, ultrasonic vibration-assisted lift-off-based patterning approach for graphene and graphene-Ag nanowire (NW) hybrid films. A 20 µm-width pattern with uniform and smooth pattern edges was neatly defined on various rigid and flexible substrates. The patterned graphene-Ag NW electrodes showed a low sheet resistance of 19 Ω/sq with a high transmittance of 93% at 550 nm, a robust stability against oxidation, and a high reliability under a bending test. The electrodes also exhibited markedly higher performance than commercial fluorine-doped tin oxide electrodes for dye-sensitized solar cells. Given its low-cost, highthroughput, and non-damaging effect, this simple and reliable patterning approach stimulates the practical applications of graphene-based flexible transparent electrodes in soft electronic and optoelectronic devices.

KEYWORDS: Flexible transparent electrode, graphene, Ag nanowires, patterning, lift-off. 1

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1. INTRODUCTION Flexible transparent conductive films (TCFs) have been extensively studied because of their applications in emerging soft electronic and optoelectronic devices, such as flexible and wearable solar cells, organic light-emitting diodes, displays, and touch-screen panels.1-5 Commercial transparent conductive oxides, such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), are unsuitable for use in flexible devices because of their brittleness even at small strains.5,6 Among potential alternatives, including conducting polymers, carbon nanotubes, graphene, metal nanowires (NWs), and hybrids of these materials, graphene-metal hybrid structures have recently attracted considerable attention owing to their high optical transparency, good electrical conductivity, and excellent mechanical flexibility.7-14 Zhu et al. demonstrated that flexible graphene-metal grid films in which a metal grid was prepared by photolithography and wet etching of metal films exhibit a sheet resistance of 20 Ω/sq with a light transmittance (T) of 90%.15 Tien et al. demonstrated that hybrid films of Ag NWs and graphene nanosheets show a sheet resistance of 86 Ω/sq (T = 80%).16 Kholmanov et al. reported that graphene films transferred on a subpercolating network of Ag NWs achieve a sheet resistance of 64 Ω/sq (T = 94%).17 Lee et al. reported a flexible graphene-Ag NW electrode with a low sheet resistance of 33 Ω/sq (T = 94%) and excellent flexibility (27% in bending strain).18 These performances surpass those of other types of TCFs. However, the complex and high-cost micropatterning process of graphene-metal composites limit their practical device applications. Electrode micropatterning is a cost- and time-consuming process, especially in the fabrication of modern and next-generation highly pixelated and arrayed devices. Micron and nanoscale patterns of graphene and graphene-metal hybrids have been produced by using direct writing and etching processes, such as focused ion beam and laser scribing.19-21 However, these 2


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methods are unsuitable in the mass production of large-area substrates. The most simple and cost-effective approach can be based on conventional photolithography, which is well established in the semiconductor industry. However, lithography is accompanied by etching processes, which are harsh to vulnerable soft materials, such as organic compounds.22,23 In general, graphene-metal hybrids are etched by a two-step etching process that involves plasma etching for graphene and wet-chemical etching for metals. O2 or H2 plasma, which is widely used for graphene etching, is detrimental to organic compounds. Strong-acid-based etchants for metal patterning can also considerably damage and contaminate organic-based active layers and substrates.15,18 Hence, the flexibility of etching processes is significantly diminished critically depending on substrate and matrix materials. Above all, the cost- and time-consuming etching of graphene-metal composites overshadows its excellences as a flexible TCF for practical device applications. Herein, we present a simple and non-damaging approach to fabricate the micron-scale patterns of graphene and graphene-Ag NW hybrid films by utilizing a lift-off technique assisted by ultrasonic vibration. The procedures are schematically described in Fig. 1. A lift-off technique is widely used for patterning various materials that are difficult to etch out and on vulnerable substrates. However, this simple and straightforward technique has yet to be employed for patterning graphene and graphene-based hybrid films despite the extensive study on graphene for the last decade. Our approach presents several advantages: (i) conventional photolithographybased patterning, which is suitable for a high throughput of large-area substrates; (ii) absence of harsh chemistry, which is applicable to any types of substrates, including vulnerable organic compounds; (iii) simple one-step process even for graphene-Ag NW hybrid films, and (iv)

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reliable micron-scale patterning, which is applicable to recent displays and touch-screen panels with 5-20 µm-width electrode arrays.24

2. EXPERIMENTAL 2.1. Samples preparation Graphene was synthesized by inductively-coupled plasma chemical vapor deposition on Cu foil (Alfa Aesar, 25 µm thick). A 500 nm layer of poly-methyl methacrylate (PMMA) was spincoated on the graphene film to serve as support during transfer. Details of the graphene synthesis and transfer were previously reported.25 Ag NWs were synthesized through a salt-mediated polyol reaction with a mixture of NaCl (99%, Sigma-Aldrich) and KBr (99%, Sigma-Aldrich). Details of the Ag NW synthesis were previously reported.26 The diameter and length of the Ag NWs were in the ranges of 20-50 nm and 30-60 µm, respectively. The Ag NWs were suspended in the mixture of water and ethanol at a ratio of 2 to 1. For graphene patterning, photoresist (PR) was patterned on desired substrates via UV photolithography with two grid-pattern masks (20 and 200 µm widths). Graphene/PMMA films were transferred on the PR-patterned substrates, as shown in Fig. 1(a). Then, the substrates were heated at 150 oC for 2 min to obtain tight contact between the graphene/PMMA layer and the opened substrate, as shown in Fig. 1(b). Finally, the samples were immersed in acetone for 10 min and subjected to ultrasonication treatment for 15 s, as shown in Fig. 1(c). The patterned samples were rinsed with isopropyl alcohol and dried using N2 gas blowing. For graphene-Ag NW patterning, an Ag NW suspension was first spin-coated on the PR-patterned substrates at 1000 rpm for 80 s and a graphene/PMMA film was subsequently transferred (Fig. 3(a)). 4


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Subsequent heating and lift-off procedures were the same as those for graphene patterning. Graphene-Ag NW patterns with 20 and 200 µm-width grids were obtained on glass and polyethylene terephthalate (PET) substrates. Dye-sensitized solar cells (DSSCs) were fabricated using patterned graphene-Ag NW electrodes as a counter electrode (CE). A working electrode was prepared on FTO glass with a 12 µm-thick mesoporous TiO2 layer and a 4 µm-thick scattering TiO2 layer. A mixed solution of 0.6 M 1-methyl-3-butylimidazolium iodide (C7H13IN2, 98%, Sigma-Aldrich), 0.03 M iodide (I2, Sigma-Aldrich), 0.10 M guanidinium thiocyanate (C2H6N4S, 99%, Sigma-Aldrich), and 0.5 M 4tert-butylpiridine (C9H13N, 96%, Sigma-Aldrich) was placed into devices as an electrolyte.

2.2. Characterization Sample images were obtained using optical microscopy (Olympus BX60MF5), scanning electron microscopy (SEM, Hitachi S-4800), and atomic force microscopy (AFM, Asylum Research MFP-3D). Light transmittance properties were characterized with an ultraviolet-visible (UV-vis) spectrophotometer (S-3100, Scinco). The samples were also characterized using an Xray photoelectron spectroscopy (XPS, Thermo Scientific MultiLab 2000 spectrometer). To measure electrical resistances, two parallel electrodes of Ti (10 nm)/Au (100 nm) were prepared on the samples through dc sputtering deposition with a shadow mask. Current-voltage properties were measured in the range of -2.5 V and +2.5 V by using a semiconductor parameter analyzer (HP4145B). For the bending test, electrodes were prepared on 20 × 20 mm2 polyethylene terephthalate (PET) substrates. The PET/electrode samples were laid between two platforms: one platform was fixed, and the other one can gradually move closer to bend the films. Electrical 5



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properties were characterized at various bending radii of curvatures, which were determined by reducing the distance between the two platforms. The photocurrent density-voltage (J–V) characteristics of the DSSCs were measured under simulated AM 1.5G sunlight at 100 mW/cm2 irradiance (Abet Technologies Sun 3000 solar simulator), which was calibrated using a silicon reference cell (HS Technologies PECSI01). The electrochemical catalytic activities of the patterned graphene-Ag NWs as a CE were characterized via electrochemical impedance spectroscopy (EIS) and a cyclic voltammetry (CV). EIS of the DSSCs was conducted under an open-circuit condition and constant light illumination (100 mW/cm2).

3. RESULTS AND DISCUSSION The as-transferred graphene/PMMA films can be overhanged on PR patterns, as shown in Fig. 2(a). To define the exact same graphene pattern as that of the opened substrate, the sample was heated above the glass transition temperature of PR. Then, the PR became pliable and reflowed. As a result, the graphene/PMMA film sagged down and touched the Si/SiO2 substrate at 120 oC. With increasing temperature, the graphene/PMMA film became more movable on the glassy PR. Moreover, owing to the glassy behavior of PMMA and the excellent flexibility of graphene, the graphene/PMMA film completely touched the opened substrate areas under its weight, as shown in Fig. 2(a). When the samples were immersed in acetone, PMMA and PR were dissolved. However, graphene over PR was still suspended freely in acetone because of the low surface tension (25.2 mN/m at 20 oC) of acetone. The suspended graphene was broken off when ultrasonication was applied. Figures 2(b) and (c) show the optical images of the grid6


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patterned graphenes with widths of 20 and 200 µm, respectively, after ultrasonic treatment. Both 20 and 200 µm patterns were neatly defined with uniform and smooth pattern edges over the whole substrate, indicating that this simple approach is applicable to recent displays and touchscreen panels with 5-20 µm-width electrode arrays. Figure 2(d) shows the SEM image of the 20 µm-patterned graphene. It indicates that the pattern edge is smooth and well-defined. The AFM result also presents the highly uniform and smooth surface of the patterned graphene areas with a root-mean-square roughness of 0.16 nm, as shown in Fig. 2(e). The Raman spectroscopy study confirmed that the crystal quality of the patterned graphenes was comparable with that of the non-patterned graphene (see Supporting Information, Fig. S1). The reliability and reproducibility of this patterning method were also confirmed through repeated patterning trials. For the patterning of graphene-Ag NW hybrid films, the procedures were the same as those for graphene-only patterning, except for spreading Ag NWs on a PR-patterned substrate, as shown in Fig. 3(a). The graphene-Ag NW hybrid films were successfully patterned on various substrates, such as rigid (Si/SiO2 wafers and glasses) and flexible (PET) substrates without any surfactant treatment or coating materials. Figures 3(b) and 3(c) show the neatly defined 200 µm grid patterns of the graphene-Ag NW hybrids on PET and glass substrates, respectively. This simple approach also successfully produced the 20 µm grid patterns of graphene-Ag NWs on PET and glass substrates, as shown in Figs. 3(d) and 3(e), respectively. Ag NWs appeared continuously and consistently on the grid patterns, whereas Ag NWs and graphene were removed completely on the opened space. The pattern edge was sharply defined without any protruding Ag NWs over the opened area as shown in Fig. 3(c). Interestingly, the mechanical agitation process, ultrasonication, is effective in cutting highly stretchable graphene and ductile Ag NWs.

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Notably, the solvent for the dispersion of Ag NWs played an important role in the uniform spread of the Ag NWs on the PR-patterned substrates. The spreading uniformity of Ag NWs is influenced by the surface tension and viscosity of the suspension solvents.27 The Ag NWs showed better dispersion in ethanol than in aqueous solutions. However, ethanol dissolved the PR easily and damaged the PR pattern. By contrast, deionized water possessed a relatively high surface tension and a low evaporation rate, which resulted in undesirable Ag NW aggregation. In the present study, we used a mixture of water and ethanol as solvent for dispersing Ag NWs. We found that the 2:1 water-to-ethanol volume ratio is the best combination for uniformly spreading Ag NWs without significant damage to PR patterns (see Supporting Information, Fig. S2). For flexible TCF applications, electrical resistance and light transmittance properties were investigated, as shown in Fig. 4. An Ag NW network film showed a low sheet resistance of 23 Ω/sq (T = 87.3%), indicating that the percolating Ag NW density exceeded the percolation threshold. We previously reported that these properties are controllable depending on the spincoating speeds of the Ag NW suspension. At higher spin-coating speeds of 2000 and 3000 rpms, 53 Ω/sq (T = 92%) and 102 Ω/sq (T = 94%) of sheet resistance and transmittance were achieved, respectively.26 These properties not only surpass those (30-80 Ω/sq with T = 90%) of ITO films but also are comparable with the reported results of Ag NW films, as shown in Fig. 4(b). Moreover, the performance of the Ag NW films was significantly enhanced by hybriding with a graphene layer and patterning. The hybriding effect of graphene and percolating Ag NWs was evidently observed in the sheet resistance, which significantly decreased to 9 Ω/sq (T = 84.6%) with a slight loss of transmittance (∆T = -2.7%). The decreased sheet resistance is attributed to the synergetic effect of the Ag NWs providing excellent conducting paths and the 8


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graphene layer covering the percolating spaces of the Ag NWs completely.17,28 Given the high sheet resistance of ~2 kΩ/sq of only a pristine graphene layer,25 the Ag NWs markedly improved the conductivity of graphene by providing effective bypaths over defects and grain boundaries of graphene. Then, the total charge transport was enhanced by both pathways of the graphene and Ag NWs. Meanwhile, the loss of transmittance was minimized because of the excellent light transmittance (97.2%) of the graphene layer. Synergetic enhancement of conductivity was most prominent in graphene/subpercolating Ag NWs, where Ag NWs cannot provide their own conductive paths but enhance conductivity of graphene. Kholmanov et al. reported a sheet resistance of 64 Ω/sq from a subpercolating network of Ag NWs covered by graphene, whereas sheet resistance of pure graphene measured 1.05 kΩ/sq.17 Synergetic effects remain effective for Ag NWs with density significantly above the percolation threshold. Lee at al. achieved a lower sheet resistance of 33 Ω/sq by using graphene/high-density percolating Ag NWs, where Ag NWs provide their own conductive paths.18 For TCF applications, however, density of Ag NWs is required to compromise with light transmittance (T > 90%). Given that an Ag NW film was patterned with 200 µm-width grids, the sheet resistance increased to 56 Ω/sq with a large increment in transmittance (94%). Eventually, the 20 µm- and 200 µm-patterned graphene-Ag NWs showed the best performance of 37 Ω/sq (T = 96%) and 19 Ω/sq (T = 93%), respectively, which is comparable with any reported result of graphene-Ag NW hybrids (Fig. 4(b)). Notably, the transmittances were virtually constant in the wide wavelength range of 400-1000 nm (Fig. 4(a)), indicating that these TCFs are useful for various applications from visible-light to near-infrared regimes.

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Another advantage of graphene-Ag NW hybrids is that the graphene layer can protect Ag NWs from oxidation and corrosion under ambient or harsh environments. Ag NWs are easily oxidized even with ambient air, causing a significant increase in contact resistance among the NWs.29,30 To investigate their long-term reliability, the graphene-Ag NW electrodes were exposed in an ambient environment for a month. The Ag NWs covered by graphene were not significantly changed in morphology after a month, whereas the Ag NWs directly exposed in air were severely oxidized, as shown in Fig. 5. Moreover, the sheet resistance (22 Ω/sq) of the 200 µm-patterned graphene-Ag NWs did not deteriorate significantly after 3 months, as shown in Fig. 6. These results indicate that the graphene layer over the Ag NWs is in tight contact with the substrate along the pattern edges, thereby effectively blocking the permeation of oxygen and moisture. Furthermore, XPS analysis of the 200 µm-patterned graphene-Ag NWs showed Ag 3d5/2 peak at 368.1 eV (see Supporting Information, Fig. S3). The binding energy agrees with the previously reported value (368.0 eV) of Ag0,31 suggesting that Ag NWs covered by graphene were not oxidized after 3 months of oxidation. A bending test was carried out to examine the reliability of the 200 µm-patterned grapheneAg NW electrodes under folding. As a counterpart, patterned graphene-Ag film samples were prepared. Ag films were deposited at various thicknesses on the PR-patterned PET substrates by a dc sputtering method, followed by graphene transfer. The 200 µm-grid patterns of the graphene-Ag film were obtained by subsequent heating and lift-off procedures as described earlier. A 7 nm-thick Ag film sample was tested because its sheet resistance (20 Ω/sq with T = 85%) was comparable with that (19 Ω/sq with T = 93%) of the patterned graphene-Ag NW sample. The experimental setup is shown in the inset of Fig. 6, where the 20 × 20 mm2 samples 10


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were bent and sheet resistance was simultaneously measured. Figure 6 shows the sheet resistance values of the patterned graphene-Ag NW and graphene-Ag film samples as functions of the distance (d) and radius (r) of the sample curvature. The sheet resistance of the patterned graphene-Ag NWs was not significantly changed until the distance decreased to 10 mm. The finding that such a stable sheet-resistance behavior under bending was maintained after 3 months of oxidation is worth noting. By contrast, the sheet resistance of the patterned graphene-Ag film started increasing once the sample was bent. The sheet resistance reached 53 Ω/sq at a distance of 10 mm. An ITO electrode on PET showed the increase in sheet resistance of more than two orders of magnitude when subjected to a similar bending test.32 Such a resistance behavior can be attributed to cracks in the electrode film at a high curvature. As an example of device applications, patterned graphene-Ag NWs on glass were demonstrated as a CE of DSSCs. The performance of this CE was compared with that of a commercial FTO electrode. To investigate their inherent characteristics, we prepared CEs without Pt deposition. Figures 7(a) shows the J–V characteristics of DSSCs with patterned graphene-Ag NWs and FTO electrodes under front-side illumination. For both front- and backside illuminations, the DSSC with the patterned graphene-Ag NWs exhibited considerably higher efficiencies (0.138% and 0.131%, respectively) compared with the DSSC with FTO (0.014% and 0.013%, respectively). Other parameters, including short-circuit current density (Jsc), opencircuit voltage (Voc), and fill factor (FF) of these DSSCs are shown in Table S1 (see supporting information). This significant improvement is attributed to the higher light transmittance and catalytic activity of graphene-Ag NWs. The patterned graphene-Ag NWs exhibited a markedly higher transmittance (93%) than FTO glass (~80% at 550 nm). Moreover, the patterned graphene-Ag NWs showed catalytic activity for the regeneration of iodide ions from triiodide 11



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ions, whereas the FTO electrode was inert in the electrolyte solution. Nyquist plots were obtained from both devices under an open-circuit condition, as shown in Fig. 7(b). The semicircle at a high frequency presents the charge-transfer resistance (Rct) at the CE/electrolyte interface. Clearly, the Rct of the patterned graphene-Ag NWs was considerably lower than that of FTO, indicating a higher catalytic activity.33,34 The DSSC with the patterned graphene-Ag NWs showed a peak of triiodide reduction to iodide at -0.26 V in a CV curve (Fig. 7(c)), suggesting an existing catalytic activity in the CE. The catalytic activities of FTO and glass are zero.35,36 Thus, the obtained peak in CV was assigned to triiodide ion reduction due to the existence of graphene. As the patterned graphene-Ag NWs were coated by Pt catalyst, solar-cell efficiency was enhanced to 7.87% (inset of Fig. 7(a)).

4. CONCLUSIONS We demonstrated a simple, ultrasonic vibration-assisted lift-off based patterning approach for graphene and graphene-Ag NW hybrid films. We successfully obtained a 20 µm-width pattern with uniform and smooth pattern edges on both rigid substrates, such as Si/SiO2 wafers and glasses, and flexible substrates, such as PET. The patterned graphene-Ag NW electrodes, which showed an excellent sheet resistance of 19 Ω/sq with an extremely high transmittance of 93% at 550 nm, were highly reliable for long-term exposure under ambient environment and for bending test. Given their superior transparent-electrode properties and interesting catalytic activity in electrolyte solution, the patterned graphene-Ag NWs showed higher performance than commercial FTO for DSSCs. These results suggest that the patterned graphene-Ag NW electrode can be used as a promising alternative to commercial TCFs for various soft electronic and 12


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optoelectronic devices, such as flexible and wearable solar cells, OLEDs, displays, and touchscreen panels. Moreover, our simple lift-off patterning approach opens up a new practical massproduction pathway for modern and next-generation highly pixelated and arrayed devices in a reliable and low-cost manner.

■ ASSOCIATED CONTENT Supporting Information Additional data, characteristic of the samples, device performance (PDF)

■ AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This study was supported by Basic Research Program through the National Research Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Education

(NRF-

2015R1D1A1A01059069). V. –D. Dao and H. –S. Choi acknowledgement support from the Korea Research Fellowship Program fund by the Ministry of Science, ICT and Future Planning through the National Research Foundation of Korea (2015H1D3A1061830).

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Figure Captions

Figure 1. Schematic of the lift-off patterning procedure of graphene: (a) As-transferred graphene/PMMA on a patterned substrate, (b) tightly contacted graphene/PMMA on opened substrate through heat treatment, and (c) lift-off patterning of graphene. Figure 2. (a) Cross-sectional SEM images of graphene/PMMA layers on PR-patterned Si/SiO2 substrates: as-transferred and heated samples at 120 oC, 140 oC, and 150 oC for 2 min. Optical images of grid-patterned graphene with widths of (b) 20 µm and (c) 200 µm. (d) SEM and (e) AFM images of 20 µm-patterned graphene. Figure 3. (a) Schematic of the lift-off patterning procedure of graphene-Ag NWs. (b) photograph and optical images and (c) SEM images of 200 µm grid patterns of graphene-Ag NWs on PET and glass substrates, respectively. (d) photograph and optical images and (e) SEM images of 20 µm grid patterns of graphene-Ag NWs on PET and glass substrates, respectively. Figure 4. (a) Transmittance spectra and (b) sheet resistance and light transmittance at 550 nm of percolating Ag NW and graphene-Ag NW hybrid films; comparison of this work with previous studies (Refs. 4,5,9,13,16,17,18,26,32). Figure 5. SEM images of Ag NWs partially covered by a graphene layer: (a) as-prepared and (b) after a month in ambient environment. Figure 6. Sheet resistance as a function of bending parameters (d and r) of patterned grapheneAg NW and graphene-Ag film electrodes.

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Figure 7. (a) J–V characteristics and (b) Nyquist plots of DSSCs with patterned graphene-Ag NWs and FTO as a CE. The inset of Fig. 7(a) shows the J–V characteristics of a DSSC with patterned graphene-Ag NW/Pt. (c) CV curve of a DSSC with graphene-Ag NWs.

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Figure 1. Schematic of the lift-off patterning procedure of graphene: (a) As-transferred graphene/PMMA on a patterned substrate, (b) tightly contacted graphene/PMMA on opened substrate through heat treatment, and (c) lift-off patterning of graphene. 564x148mm (72 x 72 DPI)



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Figure 2. (a) Cross-sectional SEM images of graphene/PMMA layers on PR-patterned Si/SiO2 substrates: astransferred and heated samples at 120 oC, 140 oC, and 150 oC for 2 min. Optical images of grid-patterned graphene with widths of (b) 20 µm and (c) 200 µm. (d) SEM and (e) AFM images of 20 µm-patterned graphene. 479x282mm (72 x 72 DPI)


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Figure 3. (a) Schematic of the lift-off patterning procedure of graphene-Ag NWs. (b) photograph and optical images and (c) SEM images of 200 µm grid patterns of graphene-Ag NWs on PET and glass substrates, respectively. (d) photograph and optical images and (e) SEM images of 20 µm grid patterns of graphene-Ag NWs on PET and glass substrates, respectively. 498x328mm (72 x 72 DPI)



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Figure 4. (a) Transmittance spectra and (b) sheet resistance and light transmittance at 550 nm of percolating Ag NW and graphene-Ag NW hybrid films; comparison of this work with previous studies (Refs. 4,5,9,13,16,17,18,26,32). 295x493mm (72 x 72 DPI)


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Figure 5. SEM images of Ag NWs partially covered by a graphene layer: (a) as-prepared and (b) after a month in ambient environment. 282x433mm (72 x 72 DPI)



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Figure 6. Sheet resistance as a function of bending parameters (d and r) of patterned graphene-Ag NW and graphene-Ag film electrodes. 423x377mm (72 x 72 DPI)


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Figure 7. (a) J–V characteristics and (b) Nyquist plots of DSSCs with patterned graphene-Ag NWs and FTO as a CE. The inset of Fig. 7(a) shows the J–V characteristics of a DSSC with patterned graphene-Ag NW/Pt. (c) CV curve of a DSSC with graphene-Ag NWs. 775x201mm (72 x 72 DPI)



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Graphical abstract 564x279mm (72 x 72 DPI)


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Simple and Reliable Lift-Off Patterning Approach for Graphene and

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