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Cu Micro-belt Network Embedded in Colorless Polyimide Substrate: Flexible Heater Platform with High Optical Transparency and Superior Mechanical Stability Ji-Hyun Lee, Doo-Young Youn, Zhenhao Luo, Ji Young Moon, Seon-Jin Choi, Chanhoon Kim, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08626 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Applied Materials & Interfaces

Cu Micro-belt Network Embedded in Colorless Polyimide Substrate: Flexible Heater Platform with High Optical Transparency and Superior Mechanical Stability Ji-Hyun Lee†, Doo-Young Youn†, Zhenhao Luo†, Ji Young Moon†, Seon-Jin Choi†,‡, Chanhoon Kim† and Il-Doo Kim*† †Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡Applied Science Research Institute, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

*Corresponding author e-mail: [email protected] KEYWORDS electrolithography, transparent electrode, flexible heater, polyimide, electrospinning

1 Environment ACS Paragon Plus


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ABSTRACT

Metal nanowires have been considered as essential components for flexible transparent conducting electrodes (TCEs) with high transparency and low sheet resistance. However, large surface roughness and high inter-wire junction resistance limit the practical use of metal wires as TCEs. Here, we report Cu micro-belt network (Cu MBN) with coalescence junction and low surface roughness for next-generation flexible TCEs. In particular, the unique embedded structure of Cu MBN in colorless polyimide (cPI) film was achieved to reduce the surface roughness as well as enhance mechanical stability. The TCEs using junction free Cu MBN embedded in cPI exhibited excellent mechanical stability up to 100,000 bending cycles, high transparency of 95.18%, and a low sheet resistance of 6.25 Ω sq-1. Highly robust Cu MBNembedded cPI based TCE showed outstanding flexible heater performance, i.e., high saturation temperature (120 °C) at very low voltage (2.3 V), owing to the high thermal stability of cPI and excellent thermal conductivity of the Cu MBN.


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Introduction Transparent conducting electrodes (TCEs) are essential components in electronics including an organic light emitting diode, touch screens, and organic solar cells. In recent years, future ubiquitous electronic devices require versatile features, such as wearability and flexibility with a superior mechanical stability.1-3 In addition, the reliable heating property of TCEs is getting much attention for various applications including transparent defroster,4 thermochromic,5 and wearable chemical sensors.6-7 However, conventional indium-doped tin oxide (ITO)-based TCEs suffer from insufficient flexibility due to their brittle and rigid nature.8-9 To meet the everincreasing requirements for the highly flexible TCEs, numerous attempts have been made to replace the conventional ITO with conducting polymers,10 graphene,11-12 carbon nanotubes,13-14 metal meshes,15-19 and metal nanowires (NWs).20-23 Among them, metal NW-based flexible TCEs have been widely studied owing to their high transparency and electrical conductivity. However, high contact resistance at metal NW intersections and large surface roughness originated from multi-stacked structures of metal NWs have hindered their practical use for TCEs. Recently, the metal mesh has also attracted considerable attention due to its high conductivity and optical transparency owing to the elimination of the wire-to-wire junction resistance. However, metal mesh fabrication processes including photo lithography11,

24

and

nanoimprint lithography25 are rather expensive and complicated multi-step processes are often required. As alternative approaches, low cost and simple fabrication processes using electrospinning lithography,17, 26-27 grain boundary lithography,16, 19 and UV-lithography28 have been introduced. Among these processes, electrospinning lithography, as one of the simplest patterning methods using electrospun polymeric nanofiber as a sacrificial hard mask, is gaining



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great attention because electrospinning method is a simple and versatile route for producing ultra-long fiber network.29-31 Besides, the conductivity and transmittance of the electrode fabricated through electrospinning lithography can be easily tuned by controlling diameter and density of as-spun hard mask fibers. As an example, Zhu et al.27 demonstrated metal mesh electrodes with high transmittance (T: 92%) and low sheet resistance (Rsh: 24 Ω sq-1) on poly(ethylene

terephthalate)

(PET)

film

via

electrospinning

lithography

by

using

polyacrylonitrile (PAN) nanofibers with diameter of 700 nm as hard mask for subsequent wetetching process. In spite of unique advantages of the electrospinning lithography, there are several limitations which should be overcome for further enhanced electrical conductivity, mechanical stability, and thermal reliability. To start with, poor bending stability of metal lines deposited on top of a plastic substrate, which is originated from poor adhesion between metal wires and a plastic substrate, leads to delamination of metal lines. Moreover, relatively high surface roughness of the metal fibers-based electrode obtained after etching process often limits the uniform interfacial contact on top of the electrode. In addition, insufficient thermal stability arising from the low glass transition temperature of conventional plastic substrates such as PET (Tg ~ 80 °C) also limits practical use, particularly for application in high-temperature resistance TCEs. Apart from conventional polymers, colorless polyimide (cPI) has high dimensional stability even at high temperature. The cPI is synthesized by the reaction between dianhydrides and diamines at an elevated temperature, known as imidization process. After imidization process, cPI shows outstanding thermal stability up to the temperature of about 300 °C. More importantly, cPI exhibits high optical transparency contrary to a commercial yellowish-brown polyimide film by suppressing the formation of charge transfer complexes, which are assisted by electronegative


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functional groups such as trifluoromethyl (–CF3), sulfone (–SO2), and ether (–O–) in the cPI chains.32 In this work, we report highly stable junction-free Cu micro-belt network (MBN) based TCEs via electrospinning lithography and subsequent embedding process in cPI film. Highly conductive junction-free Cu MBN-embedded cPI based flexible electrode (hereafter, Cu MBNcPI) provides outstanding TCE performance (T: 95.18%, Rsh: 6.25 Ω sq-1). Furthermore, we investigated potential suitability of the Cu MBN-embedded cPI based TCEs as highly robust and flexible heaters.

Material and Methods Deposition of copper and ITO thin film. The Cu thin film (300 nm) was deposited on glass (25 mm×25 mm, AMG tech.) substrate by RF sputtering at a working pressure of 5× 10 Torr with Ar gas flow rate of 5 sccm. Coating of electrospun polymer nanofibers on Cu thin film. Electrospinning was carried out using 19, 22, 25, 27% w/w Polyvinylacetate (PVAc, MW: 500 000, Sigma-Aldrich) solution dissolved in N,N-dimethylformamide (DMF) for 6 h at room temperature under stirring. The PVAc solution was loaded in a 24 mL plastic syringe attached to a syringe pump, which provided a steady flow rate of 0.1 mL/min. During electrospinning, the voltage applied to the needle was 18 kV and the collector was kept grounded. The Cu coated substrates were placed under the nozzle to collect PVAc fiber. The randomly entangled PVAc fibers were collected in 3-5 sec to control the density of the collected PVAc nanofiber. The distance from the nozzle tip to the sample surface was about 30 cm. The electrospun PVAc nanofibers-coated Cu/glass substrate was annealed in the hot DMF vapor for 30 sec at 50 °C.



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Etching of the metal film. The etching process was carried out by putting the sample in a wet etching solution, i.e., iron (Ⅲ) chloride (FeCl , 97%, Sigma-Aldrich) aqueous solution. The sample was rinsed serially with 0.003 M FeCl and 0.0015M FeCl solution, then finally the PVAc mask fibers were removed by rinsing with DMF solution. Fabrication of the Cu MBN-based TCE. This imidization process was referred by our previous study.33 Polyamic acid (PAA) solution, the precursor of cPI, was made by dissolving 1.018 g of 4,4’-(hexafluoroisopropylidene)diphthalic

anhydride

(6FDA)

and

0.569

g

of

3,3’-

diaminodiphenyl sulfone (APS) in 3.5 g of dimethylacetamide (DMAc) solvent. This solution was stirred at 500 rpm with a magnetic stirrer for 6 h at room temperature. To embed the Cu MBN within the top surface of cPI film, the homogeneously dissolved PAA solution was casted on the Cu MBN-coated glass substrate (2.5 cm × 2.5 cm) with a doctor’s blade. Then, PAA solution was imidized to cPI by heat treatment which was performed at 100, 200, and 230 °C for 30 min, 30 min and 1 h, sequentially in a box furnace. Non-embedded Cu MBN was fabricated via a slightly different process by following procedures: (1) PAA solution was coated onto a glass substrate by a doctor blade and subsequently cured at 230 °C. (2) Cu was deposited on cPI coated glass substrate by sputtering and then the PVAc solution was electrospun on the above substrate. (3) The sample was rinsed serially with FeCl3 solution, and then the PVAc mask fibers were removed by rinsing with ethanol solution. Other details are identical as those in the embedding process conditions. The ITO films were prepared by RF sputtering at working pressure of 1 × 10-2 torr with Ar gas flow rate of 20 sccm in the 150 W for 1 min at room temperature. Characterization. The transmittance of Cu MBN-cPI was measured using a UV-visible spectrometer (UV-3100 Shimadzu) with glass as a reference sample. The sheet resistance was 6 Environment ACS Paragon Plus

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measured using a four-point probe (Model 8000, MS Tech.). The optical images were taken with an Axiotech optical microscope (ZEISS) and Expert eNano printer (Enjet). All scanning electron microscopy (SEM) images were taken using Phillips (FEI). The root-mean-square (RMS) roughness was measured using atomic force microscope (SPI3800N, Seiko Instruments Inc.). The electrical and thermal properties of the heaters were examined using DC power supply equipment (34972A, Agilent) and IR camera (E8, FLIR). The bending test was carried out using a lab-made apparatus (All for the system) equipped with software for recording the number of cycles and bending speed. Results & Discussion The fabrication procedure of the Cu MBN-cPI based TCE is divided into two steps including a) synthesis of Cu MBN via electrospinning route and b) embedding process of Cu MBN within one surface of cPI substrate. The detailed fabrication procedure of the Cu MBN-cPI TCEs is illustrated in Figure 1. Firstly, UV/ozone treatment was conducted for 30 min to improve adhesion strength between thin Cu layer and underlying glass substrate before Cu deposition. Cu thin film with thickness of 100 to 300 nm is coated on a glass substrate (area of 25 mm × 25 mm) using a radio frequency (RF) magnetron sputtering. For the coating of a sacrificial mask layer, randomly distributed polymer nanofibers were electrospun on the surface of Cu-covered glass substrate. In this work, PVAc was used as a polymeric fiber mask. Since PVAc has very low glass transition temperature of 30 °C, low-temperature solvent annealing process (50 °C) enables PVAc fibers to be easily adhered to the thin Cu layer. During the annealing process, round-shaped PVAc fibers (diameter of ~750 nm) are converted to thin beltshaped structure (diameter of ~2 µm) due to the partial melting of PVAc and the fiber-fiber junctions are welded together, which are advantageous for good adhesion and reliable etching



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process. After wet etching process, the Cu lines protected by polymer nanofibers are remained and the other Cu film is entirely etched out. Then, the remained PVAc fibers were rinsed with DMF solution, resulting in Cu_MBN structure on glass substrate. In order to transfer the Cu MBN inside one top surface of cPI substrate, polyamic acid (PAA), i.e., precursor for formation of cPI, solution was directly casted on the Cu MBN coated glass substrate using a doctor blade and subsequent imidization process was sequentially carried out at 100, 200, and 230 °C. The PAA film was fully dried and imidized.34-35 Finally, the free-standing and flexible Cu MBNembedded cPI film was detached from the glass substrate after soaking in a water bath for 10 min. Figure 2a-f exhibits the morphologies and microstructures measured for each fabrication step of Cu MBN-cPI TCE. Figure 2a exhibits cross-sectional SEM image of the RF-sputtered Cu layer with the thickness of 200 nm coated on a glass substrate. The thickness of Cu layer determines the thickness (belt height) of Cu micro-belt. As a sacrificial hard mask for electrospinning lithography, the as-spun PVAc fibers exhibits the smooth surface morphology with the diameter in the range of 700–800 nm (Figure 2b). To obtain interconnected fiber-tofiber network and dramatically enhance adhesion of PVAc fibers to the underlying Cu layer, we carried out solvent annealing process using DMF vapor.27 The diameter of PVAc fibers was manipulated by varying the solvent annealing time and temperature (Figure S1). We optimized solvent annealing temperature and time as 50 °C for 30 sec to achieve stable fiber mask templates with high adhesion to the substrate. After the solvent annealing process, the diameters of micro-belt shaped fibers were increased more than two times (i.e., from 752 nm to 1.97 µm) compared to the diameter of the as-spun fiber without solvent annealing (Figure 2c). To completely remove Cu film except the Cu MBN underneath PVAc fiber templates, the sample


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was immersed in etching solution, i.e., iron chloride (FeCl3). Then, the PVAc fibers were removed by dissolving them in DMF solution. After the etching process, the diameter of Cu MBN was decreased to the diameter ranging from 800 nm to 1.15 µm compared to the diameter of the fiber mask template (2 µm), which was mainly attributed to the over-etching of Cu, socalled undercut effect (Figure 2d). Reduced diameter of Cu micro-belt is beneficial to enhanced optical transmittance. To reduce the surface roughness and improve the mechanical stability, the Cu MBN was further embedded in cPI film via PAA casting followed by imidization (Cu MBNcPI) as shown in Figure 2e. On the other hand, non-embedded Cu MBN on cPI was uneven and exposed to the substrate as shown in Figure 2f. The Cu MBN-embedded CPI TCE exhibited root mean square (RMS) roughness of 7.78 nm (Figure 2g), which is substantially lower than that (63.62 nm) of the non-embedded Cu MBN on a glass substrate, and also the peak to valley roughness (P-V) was found to be 89 nm and 625 nm, respectively (Figure S2). The Cu MBN-cPI TCE exhibited high optical transparency on a glass substrate (Figure 2h). After detachment of the Cu MBN-cPI TCE from the glass substrate, flexible and transparent Cu MBN-cPI TCE substrate was obtained (Figure 2i). Figure 3a exhibits the sheet resistance and optical transmittance of Cu MBN-cPI TCE with different thicknesses (100, 200, 300 nm) and various density of Cu MBN. The 300 nm-thick Cu MBN-CPI electrode has a tendency to exhibit low sheet resistance of 8.15 Ω sq-1 compared to the 100 and 200 nm-thick Cu MBN-cPI electrodes (61.04 Ω sq-1 and 20.3 Ω sq-1, respectively) under similar optical transmittance (~93% at 550 nm). Figure 3b shows the optical transmittance of the 300 nm thick-Cu MBN-cPI, which was measured on substrate-reference in the wavelength range from 300 to 700 nm. The result reveals that the Cu MBN films exhibits sheet resistance ranging from 8.52 to 1.29 Ω sq-1 for 300 nm-thick Cu film with the transmittance ranging from



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95.18 to 81.64%. The figure of merit (FoM) as defined by Φ= T10/Rsh (Ω-1) is generally used to evaluate the performance of TCEs.36 The Cu MBN-cPI electrode exhibited superior FoM value of 117.2 × 10-3 Ω. The Cu MBN-cPI via ESL process shows the most remarkable FoM value compared to TCEs in previous researches (See Table S2). In addition, we examined the optical properties of Cu MBN with various conditions (e.g. electrospinning time, film thickness, and kind of substrates) in supporting information. The Cu MBN visibility was tested by varying the electrospinning time (Figure S3e). All of samples were transparent to the naked eyes. However, the Cu lines prepared in longer electrospun time (above 15 s) condition are visible, whereas those of shorter time (below 15 s) are invisible despite of micrometer-sized diameter. The outstanding optoelectronic properties can be explained by combining the physical relationship for transmittance and sheet resistance as shown in equations reported in previous literature.37 = (, )

(1)

where , is bulk DC conductivity of the film and t is the thickness of metal film. In transparent conductors from nanostructured materials, the transmittance is expressed using the following equation. T = 1 + ( /2)

!

(2)

where is the impedance of free space (377 Ω) and σop is the optical conductivity (related to the absorption coefficient α as ≈ $/ ) . The absorption coefficient (α), calculated by the Brendel-Bormann model38, is 5.997 x 105 cm-1 at the wavelength of 550 nm in visible region. The thicker metal films are advantageous for TCE in terms of sheet resistance. However, the thicker film also diminishes light transmittance as shown in eq. (1) and (2). We also observed these phenomenon when we prepared film-type Cu electrodes which cover all the surfaces of


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substrates with different film thicknesses (60, 100, 200, and 300 nm) by using sputtering method (Figure S5). However, our patterned Cu MBN electrodes exhibited somewhat different performance, i.e., the distribution of samples in the graph (Figure 3a) does not follow the above theoretical relation. In the earlier literatures, Kang39 and Catrysse40 reported that the patterned metal electrodes exhibited different physical behaviors depends on their structures. In short, the electrode material (metal film) with small width and large height shows enhanced performance in terms of both transmittance and sheet resistance. In the same manner, the thicker Cu MBN film (300 nm) with same width could have improved optoelectric performances than other Cu MBN films (100 or 200 nm). In this work, the maximum thickness of Cu MBN was limited to 300 nm in consideration of economic factors (e.g., etching and sputtering time, materials cost) since there is no significant reduction of sheet resistance. To investigate mechanical stability of the Cu (300 nm) MBN-cPI TCE, the adhesion test was conducted using 3M tape. As shown in Figure 4a, the Cu MBN-cPI exhibits high mechanical stability over 400 peeling-off test cycles with small sheet resistance changes of 23.29%. On the other hand, after first adhesion test, the sheet resistance of the non-embedded Cu MBN, which is directly fabricated on cPI film, was substantially increased by 160% compared to initial sheet resistance due to the poor adhesion between the metal and substrate. Figure 4b shows a variation of the sheet resistance of the Cu MBN-cPI and an ITO film (200 nm) coated on a cPI film as a control sample with respect to the bending radius. The Cu MBN-cPI electrode exhibited stable sheet resistance even at bending radius (r) of 1 mm. In contrast, the resistance of ITO-coated cPI electrode was sharply increased at r = 9 mm. The Cu MBN-cPI also exhibited remarkably high bending stability after repeated bending test. The Cu MBN-cPI electrode exhibited no significant change of conductivity up to 100,000 cycles (r = 4 mm) without any



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fatigue damage. On the other hand, the sheet resistance of ITO-coated cPI electrode increased sharply due to the high bending stress after 50 bending cycles. The relationship between film thickness and its mechanical stability was investigated by a folding test. The films less than 10 µm showed superior flexibility but were difficult to handle. Figure S4 shows optical images of 33 µm- and 50 µm-thick cPI films after a bending cycle. They are foldable; however, the 50 µmthick film, unlike the other, exhibited a folding line and was not planar anymore in straightened state after the folding test. Given the flexibility according to thickness, optimized film thickness is in the range of 10 - 40 µm. To evaluate thermal stability, the embedded Cu MBN-cPI and nonembedded Cu MBN-cPI electrodes were placed on a hot plate at 85 °C for 100 h in air atmosphere. The embedded Cu MBN-cPI electrode showed stable sheet resistance without a rapid increase in sheet resistance during heating. On the contrary, the sheet resistance of the nonembedded Cu MBN-cPI rapidly increased after 60 h and finally showed approximately 60% increase compared to initial sheet resistance after 100 h. These excellent thermal and mechanical properties are attributed to the combination of junction-free Cu MBN and highly stable cPI matrix. Based on the high thermal stability of cPI and excellent thermal conductivity of Cu, we applied the Cu MBN based TCEs as a flexible heater. In order to protect the Cu surface from oxidation, an ITO layer (50 nm) was deposited on the top of the Cu MBN-cPI using an RF magnetron sputter at room temperature. As shown in Figure 5a, the Joule heating characteristic of the ITO-covered Cu MBN-cPI electrode was examined using an electrode having a sheet resistance of 3.9 Ω sq-1. DC bias voltage was applied between the two electrodes, and the current and the temperature were monitored. At an applied voltage of 3.0 V, the maximum temperature was 180 °C. In addition, the distribution of temperature was uniform throughout the entire film as demonstrated in IR image in the inset of Figure 5a. As shown Figure 5b, the ITO-covered Cu


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MBN-cPI TCE (sheet resistance 5.5 Ω sq-1, see Figure S7 for I-V profile) operated stably under cyclic bending stress with the bending radius in the range of 1.5 to 23.6 mm while maintaining the average temperature of 68.2 ± 2.7 oC. Since the thermal properties were unstable at high temperature (>100 oC, see Figure S8a), they were measured at an intermediate temperature. To reveal the heating behavior of the ITO-covered Cu MBN-cPI TCE, we conducted long-term stability test with various applied voltage (Figure S8b: 0 to 2.4 V in each step for 300 s). The heat generation arises uniformly without rapid change. The times to reach 90% variation in maximum temperature is 40, 36, 28, and 12 sec at 0.8, 1.2, 1.6, and 2.4 V, respectively. The high thermal stability was attributed to the structural robustness of Cu MBN-cPI by effectively preventing direct exposure of Cu micro-belts to oxidized condition. On the other hand, the nonembedded Cu MBN can be directly exposed to ambient air, which results in the degradation of electrical property due to severe oxidation and easy delamination of Cu. Consequently, the Cu MBN embedded in cPI substrate could offer the bendability, the high operating temperature, and the acceptable optical properties for flexible transparent heater compared to other types of electrodes (see Table S1). Furthermore, since the thickness of Cu MBN was thicker than those of TCEs in previous studies using electrospinning lithography process (see Table S2), the Cu MBN-cPI electrode exhibited substantially low sheet resistance and high optical transmittance as well as superior thermal stability.

Conclusion The Cu MBN based TCEs have been successfully fabricated by using electrospun polymeric nanofiber network as an etching mask template. The diameter and density of Cu-MBN were easily controlled by manipulating subsequent solvent annealing of electrospun PVAc fibers



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and electrospinning time. Due to fiber-to-fiber junction-free structure and embedding nature in colorless polyimide substrate, ultra-flat Cu MBN-embedded cPI electrode exhibited high conductivity (2.58 Ω/sq.) while maintaining high optical transmittance (88.72%) with superior FoM (117.2 x 10-3 Ω-1) after repeated bending cycles. Moreover, the Cu MBN-cPI electrode retained electrical resistance even after 100,000 bending cycles at r = 4 mm. The Cu MBN-cPI based heater generated heat up to 180 °C at 3.0 V under each bending state in the range of 24 to 1.5 mm. We demonstrated potential use of the Cu MBN-embedded cPI electrode as highly efficient flexible heater.

ASSOCIATED CONTENT Supporting Information. Figures showing additional Optical, SEM, and AFM images; Optical properties; a table showing optoelectric properties made by electrospinning lithography process. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Il-Doo Kim: 0000-0002-9970-2218 Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the Korea Institute of Machinery & Materials (KIMM) and the National Research Council of Science and Technology (NST), Republic of Korea. Wearable Platform Materials Technology Center (WMC) funded by a National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 016R1A5A1009926) REFERENCES 1.

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Figure Captions Figure 1. A schematic illustration of processing pathways of the Cu MBN: (a) electrospinning lithography (ESL) steps and (b) cPI embedding steps. Figure 2. The SEM images show the morphologies with numerical values of (a) the Cu film on glass substrate, (b) electrospun PVAc nanofibers, (c) solvent annealed PVAc nanofibers, (d) chemically etched Cu MBN, (e) Cu MBN-cPI and (f) Cu MBN on cPI. An AFM topography image of (g) Cu MBN-cPI. The photographs show (h) transparent Cu MBN on glass substrate and (i) flexible and transparent Cu MBN-cPI. Figure 3. (a) Optical transmission (at 550nm) versus sheet resistance properties of the Cu MBN with thickness of 100, 200, and 300 nm. (b) Optical transmittances of the Cu MBN electrodes having best performance. Figure 4. (a) Adhesion test (3M tape test) of the Cu MBN on cPI and Cu MBN-cPI, (b) bending test according to bending radius of the Cu MBN-cPI and ITO on cPI (c) bending cyclic test versus bending cycles of the Cu MBN and ITO on cPI and the Cu MBN-cPI with a bending radius of about 4 mm, and (d) thermal stability test of the Cu MBN non-embedded in cPI and Cu MBN-PI at 85 oC. Figure 5. (a) Current and temperature distribution of the ITO-covered Cu MBN-cPI TCE by Joule heating under diverse voltages. Insets show infrared (IR) camera images depending on each applied voltage (b) Current and temperature of the embedded Cu MBN according to bending radius. Insets show IR camera images of each bending radius (1.5, 3, 23.6 mm) with temperature.



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Cu Microbelt Network Embedded in Colorless ... - ACS Publications

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