Transparent, Flexible Heater Based on Hybrid 2D Platform of

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Surfaces, Interfaces, and Applications

Transparent, Flexible Heater Based on Hybrid 2D Platform of Graphene and Dry-spun Carbon Nanotubes Luhe Li, Soon Kyu Hong, Yeongsu Jo, Mengdi Tian, Chae Young Woo, Soo-Hyung Kim, Jong-Man Kim, and Hyung-Woo Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02225 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Transparent, Flexible Heater Based on Hybrid 2D Platform of Graphene and Dry-spun Carbon Nanotubes Luhe Li, † Soon Kyu Hong, † Yeongsu Jo, § Mengdi Tian, † Chae Young Woo, † Soo Hyung Kim,†,‡,§ Jong-Man Kim,†,‡,§,* Hyung Woo Lee†,‡,§,* †Department §Research

of Nano Fusion Technology, ‡Department of Nanoenergy Engineering and

Center of Energy Convergence Technology, Pusan National University, Busandaehakro 63beon-gil 2, Geumjeong-gu, Busan, 46241, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT

A high-performance, flexible, and transparent heater based on a hybrid of dry-spun carbon nanotubes (CNT), which is pulled out directly from a super-vertically aligned CNT forest, and graphene is fabricated. The electrical, optical, and electromechanical properties of two different kinds of hybrid devices—graphene above or below the CNT film, and simple CNT film heating devices that are made of 1 or 2 layers of CNTs—are studied. The results prove that the hybrid structure film heaters are superior to the simple CNT film heaters. The simple single-layer CNT film and double-layer CNT film heaters attain maximum temperatures of 48 ℃ and 64 ℃ with transmittances of 73 % and 64 % at a wavelength of 550 nm, respectively, whereas the Singlelayer CNT sheet/Graphene/PET and Graphene/Single-layer CNT sheet/PET hybrid heaters attain maximum temperatures of 81 ℃ and 85 ℃ with transmittances of 71 % and 68 %, respectively. The 10,000 bending cycle test suggests that the Graphene/Single-layer CNT sheet/PET heater has good mechanical and thermal stability. Further, defrost test and portable heating with a 9 V battery prove the possibility of using the hybrid heater for vehicle defrosting, portable heating and wearable devices.

KEYWORDS: dry-spun CNT film, graphene, hybrid platform, transparent, flexible heater


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1. INTRODUCTION Transparent and flexible film heaters have attracted many researchers’ interest because of the practical need for rapid defrosting of vehicles and fast response of electric screens in severe environments. Thin indium tin oxide (ITO) films have been widely used as heating elements because of their high transmittance and low resistance.1,2 However, ITO film heaters have some limitations. ITO films contain the rare metal indium and the fabrication methods such as physical– chemical vapor deposition or magnetron sputtering renders the ITO films expensive. Besides, ITO films are brittle and thus unsuitable for flexible devices.3-7 Therefore, other conductive and flexible materials have been explored to replace ITO as a heating element. The representative materials are metal nanowires,5,8-12 carbon nanotubes (CNT),5,7,13-17 and graphene.4,18,19 However, these materials also have some limitations. Because of the randomly aligned thin films, which are fabricated by spin coating or spraying metal nanowires or CNT solutions, the high contact resistance at the tube-tube junctions and the micrometer-wide holes within the random networks that act as non-conductive open spaces cause performance degradation of the film heater with high resistance. To decrease the resistance, common approaches such as increasing the density or the weight ratio of metal nanowires were introduced, even though they decrease the transmittance.1722

In addition, since metal nanowires easily oxidize when exposed to high-temperature and high-

humidity conditions, which is the working environment of a vehicle defroster, the resistance drastically increases, resulting in a reduction in its service life.23 Graphene is the most promising material to replace ITO as a transparent conductive film (TCF) because of its theoretically high transparency and high conductivity; however, it is difficult to obtain high-quality graphene because of its polycrystalline structure and various defects (grain boundaries, ripples, wrinkles, folds, and cracks).24-27 Recently, some researchers have tried to hybridize graphene with various metal


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nanowires or metal grids by spinning, electrohydrodynamic jet printing, photolithography, or other methods to achieve low-sheet resistance and high-transmittance TCFs to replace commercial ITO in a broad range of applications.27-33 Nevertheless, these methods still cannot avoid the oxidization of metals. In 2002, Fan’s group first reported the novel super-aligned CNT forests that can be pulled out as a CNT sheet and successfully achieved a 4-inch wafer-scale synthesis of superaligned CNT arrays.34 Some researchers have tried to use this technique to fabricate transparent and flexible heaters in which multilayer CNT sheets and metal particles were used to reduce the high sheet resistance; however, these methods lowered the transmittance of the heaters.7,16-17 Here, we report a transparent, flexible, and lightweight thin film heating device based on the hybrid platforms of graphene and CNT sheets by a novel approach using the dry spinning technique. Different hybrid structures such as Graphene/Single-layer CNT sheet/PET (Graphene/S-CNT/PET) and Single-layer CNT sheet/Graphene/PET (S-CNT/Graphene/PET), and simple CNT sheet thin films such as Single-layer CNT sheet/PET (S-CNT/PET) and Double-layer CNT sheet/PET (D-CNT/PET) were studied. The time-temperature profiles and heat distribution analyses showed that the performance of the hybrid film heaters is superior to that of the simple CNT film heaters: at the same applied voltage, the hybrid platform heaters achieved a higher temperature while maintaining a high transmittance. The bending test results showed that the hybrid platform heater is stable even after 10,000 cycles. In addition, long-term operation tests showed that the heater temperature remains stable for four hours of continuous heating. Further, based on the demonstration of the defrosting and portable heating test with a 9 V battery, the hybrid platform heater is expected to be suitable for vehicle defrosting, smart windows, portable heating, smart wearable devices, etc.


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2. EXPERIMENTAL SECTION Synthesis of super-vertically aligned MWCNT array. Super-vertically aligned multiwalled carbon nanotubes (MWCNT) arrays were synthesized in a low-pressure chemical vapor deposition (LPCVD) tube furnace (UP35A, YOKOGAWA ELECTRIC KOREA CO., LTD.). As a catalytic layer for growing CNTs, 3 nm iron was deposited on a Si substrate with a 200 nm oxide layer by electron beam (e-beam) evaporation at a rate of 0.3 Å/s. The thickness and evaporation rate were monitored using a crystal sensor in an evaporation chamber. For the growth, the substrate was first loaded in the center of the chamber. The temperature of the furnace was then increased to 520 ℃ in 26 min under 400 sccm of Ar gas. Then, 500 sccm of H2 gas was introduced into the chamber for 1 min to allow the catalyst film to form catalyst particles with appropriate sizes. After the pretreatment process, the temperature was further increased to 700 ℃ in 4 min under 400 sccm of Ar gas. Then, 200 sccm of C2H2 and 400 sccm of H2 were introduced into the chamber for 30 min to grow CNTs. After the growth process, the furnace was allowed to cool to room temperature for 20 min under 400 sccm of Ar gas.

Synthesis of graphene. The graphene sheet was also synthesized by LPCVD. A high-purity copper foil was used as a substrate and was carefully loaded at the center of the furnace. The temperature of the furnace was first increased to 1050 ℃ in 1 h under 30 sccm of H2 and allowed to stabilize for 20 min. Then, 50 sccm of CH4 was introduced into the furnace as a carbon source to synthesize graphene for 30 min. Finally, the furnace was allowed to cool to room temperature for 40 min under 30 sccm of H2.


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Preparation of 2D platform hybrid heaters. The simple 2D platform CNT heaters were fabricated by the dry spinning technique, and the CNT sheets were drawn out directly from the super-vertically aligned CNT array.35,36 Figure 1 shows the schematic design of the hybrid 2D platform heaters. Figure 1a shows the transfer process of graphene. A PMMA solution (9 wt%) was spin coated on graphene at 900 rpm, following which the copper substrate was removed using a commercial copper etchant (SIGMA-ALDRICH). Then, the PMMA-supported graphene was washed in the deionized water to remove the copper etchant. The synthesis process of the hybrid Graphene/S-CNT/PET film heater is shown in Figure 1b.

Figure 1. Schematic diagram of preparation of hybrid 2D platform heaters: (a) wet transfer of graphene; 9 wt% PMMA was spin coated as the support layer and Cu was removed by the copper etchant, and fabrication process of (b) Graphene/S-CNT/PET; graphene was transferred on the pre-prepared CNT film and etched in acetone to remove PMMA, and (c) S-CNT/Graphene/PET; CNT film was transferred on the pre-etched (PMMA free) graphene by drawing out directly from the super aligned CNT array.


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The CNT sheet was first drawn out from the CNT array and transferred on a PET substrate and several drops of IPA were carefully dropped on the CNT sheet to make CNT sheet dense. Finally, the CNT sheet was strongly attached to the PET substrate. Then, the PMMA-supported graphene was placed onto the S-CNT/PET and immersed in acetone for 20 min at 60 ℃ to remove the PMMA. The synthesis process of the hybrid S-CNT/Graphene/PET heater is shown in Figure 1c. The PMMA-supported graphene was first placed onto the PET substrate and etched in acetone to remove PMMA. Then, the CNT sheet was transferred by drawing from the CNT array. Ag paste was used as electrodes and the size of the heater was fixed at 2 cm × 1.5 cm. The temperatures of the 2D platform heaters were determined using an infrared camera (T630SC, FLIR® Systems, Inc.).

3. RESULTS AND DISSCUSSION Characterization of CNT and graphene. The synthesized CNT and dry-drawn CNT sheets were analyzed by field emission scanning electron microscopy (FE-SEM, SUPRA40VP) and field emission transmission electron microscopy (FE-TEM, TALOS F200X), and the results are shown in Figure 2. The length of the spinnable CNT array was about 280–300 μm and the CNT array was highly perpendicular to the substrate (Figure 2a). The high-magnification cross-view FE-SEM image of the spinnable CNT array is shown in Figure 2b. As can be seen, the CNT bundles were parallel to each other and highly oriented, which because of the density of the catalyst is quite high. The highly dense CNT bundles not only ensure that the CNT array grow uniformly and vertically to the substrate, but also induce strong van der Waals’ forces between the adjacent CNT bundles, which is important for pulling out the CNT sheet.34,37 The spinnable CNT has a diameter of about 20 nm and 10 walls, and the outermost wall contains some amorphous carbon particles, which can be seen in the FE-TEM image in Figure 2c (the inset shows the CNT on the copper


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grid). The quality of CNT array can be further improved by hetero-atom doping and ammonia etching which will increase the conductivity of the CNTs.38-40 The top-view FE-SEM image of the dry-spun CNT sheet that was directly drawn from the super-aligned CNT array is shown in Figure 2d. Micrometer-wide spaces between the CNT sheets can be clearly observed, and these nonconductive spaces will significantly increase the resistance of the CNT sheet. Dipping into a volatile organic solvent or increasing the number of CNT sheets are common strategies to decrease the resistance of the CNT sheet, despite the limitation of decrease in CNT sheet transmittance associated with these approaches. To decrease the resistance of the CNT sheet while maintaining a high transmittance, we propose a hybrid of graphene and CNT sheet. The experimental results show that our strategy is quite effective, which will be discussed later.

Figure 2. (a) Low- and (b) high-magnification cross-view FE-SEM images of super-aligned CNT array; the length of the CNT array is about 300 μm and the CNT bundles are parallel with each other, (c) FE-TEM image of CNTs (inset shows the CNT on the copper grid); the diameter of the CNT is about 20 nm and the outside contains some amorous carbon particles, and (d) top-view FE-SEM image of CNT sheet which was drawn from the CNT array shown as CNT film is sparse and not uniform.


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The Raman spectra of the synthesized CNTs and graphene are shown in Figure S1. The typical D band and G band, which are located at around 1333 cm-1 and 1581 cm-1, are observed in the Raman spectrum of CNT shown in Figure S1a. The D band is usually attributed to the presence of amorphous carbon, surface defects, or lattice imperfection, and reflects the disordered features of multi-walled CNTs (MWCNTs). The G band, or the “graphite” band, is generally generated by the in-plane vibration of the sp2 carbon atom, and the corresponding vibration mode has E2g symmetry.41-44 The ID/IG ratio, which reflects the degree of nanotube defects, is approximately 0.92 and shows that the CNT is well graphitized. The Raman spectrum of the graphene sheet shown in Figure S1b displays two most intense features: the G peak at 1580 cm-1 and the 2D peak at 2680 cm-1. The IG/I2D ratio is approximately 1/3, which proves that the synthesized graphene is a single layer.45,46 Thus, the single-layer graphene is expected to greatly improve the conductivity of the CNT sheet.

Thermal and electrical properties of 2D platform heaters. Figure 3 shows the temperature-time plots of 2D platform heaters at 12 V during on-off operation. To accurately record the temperature of the heaters, the infrared camera was first turned on, and the DC power was turned on after 5 s. After turning on the DC power, the temperature of the heaters rapidly increased to the maximum temperature and remained stable at the saturation temperature. The hybrid 2D platform heaters showed higher temperatures than that of the simple CNT film heaters. Graphene/S-CNT/PET attained a higher temperature (89 ℃) than S-CNT/Graphene/PET did (81 ℃). On the other hand, among the simple CNT film heaters, D-CNT/PET attained a higher temperature (69 ℃) than the S-CNT/PET heater did (54 ℃). After heating for further 75 s, the DC power was turned off, and the temperature of the heaters decreased to room temperature in 30 s. The on-off temperature-time


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plots show that the 2D platform heaters have fast response feature, and the temperatures of the 2D platform heaters are quite stable. In addition, no significant temperature fluctuation (± 2 ℃) was observed during the test.

Figure 3. Temperature-time plots of 2D platform heaters at 12 V during on-off operation. After turning on the power, the temperature was increased to the saturation temperature in short time and kept stable. And it was cooled to the room temperature rapidly after turning off the power.

Figure S2 shows the corresponding maximum temperature versus time curves of the 2D platform heaters at 6, 9, and 12 V. At low applied voltages of 6, 9, and 12 V, the maximum temperatures of the simple S-CNT/PET heater were 31, 43, and 54 ℃, while that of the D-CNT/PET heater were 40, 52, and 69 ℃, respectively. For the hybrid film heaters, the maximum temperatures were 43, 56, and 81 ℃ for S-CNT/Graphene/PET, and 47, 65, and 89 ℃ for Graphene/S-CNT/PET, respectively. All the 2D platform heaters attained the maximum temperature in 25 s, which show


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the fast response feature of these devices. Besides, the temperature-time curves confirm the highly stable feature of the hybrid film heaters; The hybrid films maintained an almost constant temperature after attaining the maximum temperature, while the simple CNT heaters showed lower stability and larger temperature fluctuations after attaining the maximum temperature, which might result from non-uniform thermal energy diffusion. The average maximum temperature of the 2D platform heaters at 6, 9, and 12 V, which were measured for over 10 samples, and the sheet resistance of the 2D platform heaters that were measure by the two-point probe method,47,48 are shown in Figure 4. Figure 4a shows the average maximum temperature of the 2D platform heaters. At a low applied voltage of 6 V, the simple CNT film heaters S-CNT/PET and D-CNT/PET attained maximum temperatures of 30.64 ± 2.32 ℃ and 35.92 ± 2.06 ℃, respectively, whereas the hybrid film heaters S-CNT/Graphene/PET and Graphene/S-CNT/PET attained maximum temperatures of 42 ± 1.08 ℃ and 43.08 ± 1.21 ℃, respectively. At a higher applied voltage of 9 V, S-CNT/PET and D-CNT/PET attained maximum temperatures of 36.9 ± 3.08 ℃ and 47.64 ± 4.72 ℃, respectively, whereas S-CNT/Graphene/PET and Graphene/S-CNT/PET attained maximum temperatures of 60.42 ± 1.85 ℃ and 63.31 ± 1.90 ℃, respectively. When the applied voltage was further increased to 12 V, the average maximum temperatures of S-CNT/PET, D-CNT/PET, S-CNT/Graphene/PET, and Graphene/S-CNT/PET increased to 47.50 ± 6.10, 63.62 ± 8.06, 80.67 ± 2.36, and 85.38 ± 2.73 ℃, respectively. In summary, the hybrid 2D platform film heaters showed higher temperatures than that of the simple CNT film heaters under the same applied voltage. Among the simple CNT film heaters, DCNT/PET showed a higher maximum temperature than that of S-CNT/PET, whereas for the hybrid film heaters, Graphene/S-CNT/PET showed a higher maximum temperature than that of SCNT/Graphene/PET.


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Figure 4. (a) Maximum temperature of 2D platform heaters at 6, 9, and 12 V, and (b) sheet resistance of 2D platform heaters. Hybrid film heaters showed higher temperature and lower sheet resistance than the simply CNT film heaters at the same voltage. Specially, when graphene is on the top of CNT, it has higher temperature and lower sheet resistance than graphene is under the CNT film.

Figure 4b shows the average sheet resistance of the 2D platform heaters. Distinctly, the singlelayer CNT film showed the highest sheet resistance of about 1065.27 ± 316.96 Ω/sq, while that of the sheet resistance of the double-layer CNT film decreased by about 50% to 562.90 ± 157.79


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Ω/sq, and the monolayer graphene showed a relative lower sheet resistance (826 ± 133 Ω/sq) than the single-layer CNT which is still higher than the hybrid film. For the hybrid films, Graphene/SCNT/PET showed a lower sheet resistance (298.19 ± 22.19 Ω/sq) than that of SCNT/Graphene/PET (334.36 ± 19.12 Ω/sq). The resistances of S-CNT/Graphene/PET and Graphene/S-CNT/PET decreased by about 68 % and 72 %, respectively, compared with that of the single-layer CNT film. In conclusion, the hybrid film showed a lower sheet resistance than that of the simple CNT film and the monolayer graphene. Among the simple CNT films, D-CNT/PET showed a lower sheet resistance than that of S-CNT/PET, and among the hybrid films, Graphene/S-CNT/PET showed a lower resistance than that of S-CNT/Graphene/PET. The different fabricate methods were also taken to compare the electrical performance of the hybrid film (Figure S3). The samples made by dry-spun method show much lower resistance than the spin-coating method which prove the unique advantage of the dry-spun method for the hybrid films. According to the Ohm’s law and the Joule’s law, the produced thermal energy and the power of the heaters can be calculated by Q = V2t/R and P = V2/R, where Q is the thermal energy produced by the device, V is the voltage applied to the device, t is the working time, R is the resistance of the device, and P is the power. Clearly, the thermal energy produced by the heater at a certain time is related to the applied voltage and the resistance of the heater. The higher the applied voltage and the lower the resistance, the more is the thermal energy generated, which implies that a higher temperature will be reached in a certain environment. The theoretical conclusion is consistent with the experimental results in that the hybrid-structured film heater has a lower resistance and attains a higher temperature compared with the simple CNT film heater under the same applied voltage.


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Figure 5. FE-SEM images of (a) S-CNT/PET, (b) D-CNT/PET, (c) S-CNT/Graphene/PET, and (d) Graphene/S-CNT/PET (the insets show the schematic diagrams of each 2D platform heaters). Double-layer CNT film showed fewer non-conductive holes than the single layer CNT film, and graphene covered all of the non-conductive holes in the hybrid structures.

The morphology of the 2D platform heaters were characterized by FE-SEM, and the results are shown in Figure 5. The insets show the schematics of the 2D platform heaters. As shown in the FE-SEM image of the S-CNT/PET film in Figure 5a, vacant spaces exist between the CNT bundles. This non-uniform CNT film was caused by the dry-spinning process, as observed in the FE-SEM images of the in situ dry-spun CNT film in Figure 2d. These vacant spaces between the CNT bundles will significantly increase the resistance of the single-layer CNT film. In the case of D-CNT/PET film, the CNT bundles are entangled with each other and the vacant space has a relatively smaller area, as shown in Figure 5b. The more the CNT layers are stacked by the dry spinning process repeatedly, the lesser is the vacant space generated in the CNT film.


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Besides, the additional CNT film layers increased the connection between CNTs, which is beneficial for improving the conductivity of the heater.49 Compared with the simple CNT device, the hybrid film device showed a lower resistance. The main reason is believed to be the covering of all the non-conductive micrometer-wide holes within the random networks by graphene, which can be confirmed from Figure 5c and Figure 5d. Figure 5c shows the FE-SEM image of S-CNT/Graphene/PET. Micrometer-wide holes can still be seen between the CNT bundles; however, these original non-conductive micrometer-wide holes are covered with graphene. Compared with the original single-layer CNT film, the sheet resistance of S-CNT/Graphene/PET decreased by about 68% mainly because the additional graphene layer not only covered the non-conductive bare spaces between the CNT bundles, but also increased the amount of electron-transfer tunnels between the CNT and the graphene layer. Figure 5d shows the FE-SEM image of Graphene/S-CNT/PET. As can be seen, the graphene is strongly adhered to the bottom CNT film (a crack in the top graphene film that was made by rubbing on the surface of wipers is shown in Figure S4a, which clearly shows the presence of graphene). The average sheet resistance of Graphene/S-CNT/PET is 298.19 Ω/sq, which is only 28% of that of S-CNT/PET and lower than that of S-CNT/Graphene/PET. When the CNT film is under the top graphene layer, it will be completely wrapped by graphene, which will result in the creation of more electron-transfer tunnels compared with that in S-CNT/Graphene/PET. Besides, the top graphene layer helps decrease the thickness of the CNT layer, resulting in a denser hybrid film. Furthermore, the reported MWCNT sheet acts as a “soft” nonplanar support that will release the structural distortions of graphene, thereby giving rise to a graphene layer with no noticeable wrinkles, which will improve the conductivity of graphene, are also observed50. As can be seen in Figure 5d, no noticeable wrinkles are observed when graphene covers the CNT surface. However,


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noticeable wrinkles are observed when graphene is under the CNT film, as shown in Figure S4b. The wrinkles in the graphene layer are considered a kind of defect that will decrease the conductivity of the graphene film,51 and are also one of the reasons for the lower resistance of Graphene/S-CNT/PET than that of S-CNT/Graphene/PET. Atomic force microscopy (AFM, Park NX10, Park System) was performed to further verify the effect of different structures on the morphology of these devices, and the AFM images are shown in Figure 6 (scan areas are 10 μm × 10 μm, the inner blacks lines represent the roughness fluctuations corresponding to the straight yellow lines, and the black lines are all in the same scale). The AFM image of S-CNT/PET in Figure 6a shows that the CNT bundles are sparsely dispersed on the PET substrate and contribute to the roughness, as shown from the fluctuation. The AFM image in Figure 6b shows that the density of the CNT bundles of D-CNT/PET has increased and all the bundles are entangled each other, resulting in a high surface fluctuation. There is also a possibility that many bundles and spaces of CNTs as increasing the number of layer cause the nonuniform surface resulting in some areas having many CNTs “mountain” while some areas not being covered by the CNT film forming “basin”, thus the D-CNT/PET film heater has the roughest surface arisen from the non-uniform CNTs transferred by the dry spinning process. In the case of S-CNT/Graphene/PET, a similar morphology and fluctuation peak as that of S-CNT/PET can be seen from the AFM image in Figure 6c. However, the Graphene/S-CNT/PET heater shows a different surface profile with lower surface roughness and fluctuation than those of any other structures, as shown in the AFM image in Figure 6d.


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Figure 6. AFM images of (a) S-CNT/PET, (b) D-CNT/PET, (c) S-CNT/Graphene/PET, and (d) Graphene/S-CNT/PET (Scan area is 10 μm × 10 μm, the inner black lines represent the roughness fluctuations corresponding to the straight yellow lines, and the black lines are all in the same scale). Double layers CNT film shows the highest surface fluctuations and Graphene/S-CNT/PET shows the lowest surface fluctuations, which proves that the top graphene layer wraps the CNT bundles and makes the film denser.

No significant fluctuation was observed anywhere, not even at the CNT bundles. This implies that the covered graphene layer strongly adhered to the CNT and/or converted the raised large bundles into a flat dense CNT film. This phenomenon is expected to be one of the reasons for the greatly reduced resistance of the hybrid heater. The results are consistent with the dynamic static data of the AFM images, which are shown in Table S1. The surface roughness of Graphene/SCNT/PET is 13.461 nm, which is quite lower than that of the other devices (20.829, 33.182, and


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24.426 nm for S-CNT/PET, D-CNT/PET, and S-CNT/Graphene/PET, respectively). The AFM images proved that when graphene covers the CNT, it would bond all the CNT bundles and render the CNT film denser. Moreover, it helps decrease the resistance by increasing the density of the CNT connection, which will decrease the tube-tube contact resistance. In summary, the top graphene layer not only covers all the non-conductive holes and provides additional electron transfer tunnels, but also makes the hybrid film denser, which will decrease the tube-tube contact resistance and improve the conductive properties of the hybrid film.

Optical properties of 2D platform heaters. The transmittance of the 2D platform heaters in the visible light range of 350–800 nm was measured using a UV-visible spectroscope (Evolution 300 UV-VIS), and the results are shown in Figure 7 (the inset shows the photographs of 2D platform heaters with the scale bar of 1 cm). A bare PET thin film was measured as a reference and showed the highest transmittance of 82.79 % at a wavelength of 550 nm. The transmittances of the hybrid heaters are lower than that of S-CNT/PET but higher than that of D-CNT/PET, which is mainly attributed to the higher transmittance of a single graphene layer than that of a single CNT layer obtained by the dry spinning process. For the simple CNT films, the transmittance of the single layer is 73.3 % and that of the double layer is 64.05 %, which is quite lower than that of monolayer graphene on PET (79.45 %).52 The transmittances of the hybrid film heaters are similar, with Graphene/S-CNT/PET showing a slightly higher transmittance (71.15 %) than that of SCNT/Graphene/PET (67.69 %).


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Figure 7. Transmittance of a PET film and 2D platform heaters on a PET film (the insets show the photographs of 2D platform heaters with the scale bar of 1 cm). Single layer CNT film shows higher transmittance than other devices, and graphene on top of CNT film shows higher transmittance than under the CNT film. Double layers CNT film device shows the lowest transmittance in the visible light range.

The transmittance of the hybrid films can be calculated by Ttotal = T1 × T2, where T1 and T2 are the transmittances of film 1 and film 2, respectively, and Ttotal is the transmittance of the hybrid film.53,54 According to this formula, the transmittance of the hybrid film and the double-layer CNT film should be about 71.39 % and 64.39 %, respectively. The calculated results are mostly consistent with the experiment results, while the transmittance of S-CNT/Graphene/PET is lower than the calculated result, which may be related to the surface roughness of the composite films. The S-CNT/Graphene/PET was found to be much rougher than the PET/S-CNT/Graphene (Table S1) and therefore would be more sensitive to the light scattering, hindering transmission of larger


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amount of incident light through the device. This is likely to be a main contributor to the discrepancies between the theoretical and experimental transmittances. The relatively low transmittance of the devices is caused by the low transmittance of the bare PET film. The transmittance of the devices in visible range when using PET film as baseline (the transmittance of a PET film is 100 %) is shown in Figure S5. The transmittance of the single layer CNT film and double layer CNT film are 88.08 % and 77.03 %, while the hybrid films show higher transmittance that are 84.95 % and 82.86 % when graphene above and under the single layer CNT film respectively.

Figure 8. Temperature of Graphene/S-CNT/PET (a) during 10,000 bending cycles with 3.2 mm of bending curvature, and (b) according to the bending radius during the bending cycles.


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Mechanical and thermal stability of 2D platform heaters. Figure 8a shows the high thermal stability of Graphene/S-CNT/PET with repeated bending cycles at 12 V. The hybrid film was bent into a circle, corresponding to a bending radius of about 3.2 mm. The temperature was measured after every 1,000 bending cycles for 10,000 bending cycles using an infrared camera. The initial temperature of the device was 81 ℃, and the temperature was slightly decreased to 78.8 ℃ after bending for 10,000 cycles which is just about 2% change from the initial temperature. After bending for 10,000 cycles, the sample still showed uniform thermal distribution. Figure 8b shows the temperature of the heater according to the bending radius. The temperature of the device was measured from the initial state (20 mm) to the maximum bent state (3.2 mm), and to the initial state again. During the bending cycles, the temperature of the device was slightly decreased when it was bent, then returned to the initial temperature when it came back to the initial state which proved the resistance is almost the same during one bending cycle and it shows the excellent flexibility of the device. In order to show the mechanical stability of the hybrid films, we performed adhesion test using the 3M tape as shown in Figure S6. After attaching and detaching for 10 times, the resistance of the S-CNT/Graphene/PET was increased about 4.5 times which is mainly because the top CNT layer was removed by the tape. While the sheet resistance of the Graphene/SCNT/PET was increased only about 0.3 times in comparison with the initial resistance. These bending and adhesion test results have proved that the substrate of Graphene/S-CNT/PET has good mechanical and thermal stability as a smart wearable device in the future.

4. APPLICATIONS


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To apply as a defrosting system, a defrost test was performed for the 2D platform heaters. A transparent glass (75 mm × 25 mm × 1.1 mm) was firstly immersed in the liquid nitrogen for 20 s, it was then taken out and heated using the heaters with a 12 V DC power.

Figure 9. Photographs of defrosted results for heating duration according to the various 2D platform heaters. Graphene/S-CNT/PET shows the fastest defrosting time and the S-CNT/PET shows the slowest defrosting time at the same voltage.

Figure 9 shows the photographs of the glasses with 2D platform heaters during the defrost test. As shown in Figure 9, before heating with the 2D platform heaters, the word under the slide glass cannot be observed, and after heating for 25 s, the Graphene/S-CNT/PET firstly partly defrosted and the word can be observed. After heating for 50 s, the frost on the slide glass heated with the Graphene/S-CNT/PET has almost gone, and the slide glass heated with the S-CNT/Graphene/PET shows relative smaller defrosted area, while there is no obvious defrosted part for the glass which heated by the simple CNT film heaters. After further heating for 25 s, the frost on the slide glass


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heated with the hybrid film heater has gone, and the slide glass heated with D-CNT/PET showed large defrosted area than the S-CNT/PET. The defrost test showed the Graphene/S-CNT/PET has good defrost performance and can be used for the vehicle defrosting and other areas.

Figure 10. Infrared camera images of a large Graphene/S-CNT/PET film (4.5 cm × 4.5 cm) attached on the back of a hand in (a) grab, (b) relax, and (c) spread states before and after turning on 9 V battery. The temperature is increased and maintained after turning on the power during the grab, relax and spread states.

A large hybrid Graphene/S-CNT/PET film heater was also fabricated to show its applicability as a wearable device and potential for smart wearable clothes, smart windows, and mobile defrosting. To show the possibility of a wearable heating system, the large (4.5 cm × 4.5 cm)


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hybrid film heater was fixed on the back of a hand using a rubber band. The stretching of the heater attached to the back of the hand during the grabbing, stretching, and spreading actions can be seen from the infrared camera images shown in Figure 10. Before turning on the 9 V battery, the maximum temperature of the heater was 36.3 ℃, which is the same as that of skin during the grabbing, stretching, and spreading actions. After turning on the power, the temperature increased to about 44.5 ℃. During the grabbing, relaxing, and spreading actions, the heater maintained its temperature at about 44.5 ℃. Moreover, as the hybrid film is lightweight and flexible, thus there is no disturbance on the dexterity of action. To show the possibility of portable heating using the Graphene/S-CNT/PET heater, a 9 V DC power was used as a power supply and the temperature of the heater was measured for a long period. The results are shown in Figure S7 (the insets show the infrared images the heater after heating for 2 and 4 h). The test results show that the temperature of the Graphene/S-CNT/PET is quite stable (68 ± 1 ℃) during the prolonged operation and the lower working voltage proved the possibility of heating with a portable power supply.

5. CONCLUSIONS In this paper, we report for the first time a flexible, transparent, and lightweight film heater based on a hybrid platform of graphene and CNT prepared using the dry spinning technique. The electrical, optical, thermal, and mechanical stabilities of the hybrid and simple CNT film 2D platform heaters were investigated. The S-CNT/PET and D-CNT/PET film heaters attained maximum temperatures of 48 ℃ and 64 ℃ with transmittances of 73 % and 64 % at 550 nm, respectively. On the other hand, the S-CNT/Graphene/PET and Graphene/S-CNT/PET platform film heaters attained maximum temperatures of 81 ℃ and 85 ℃, which are higher than that of the


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simple CNT film heater, and high transmittances of 68 % and 71 %, respectively. Compared with the simple CNT heaters, the hybrid film heaters showed higher temperatures because of their low resistances, which is mainly because of the additional graphene layer that covered all the nonconductive holes in the uniform CNT film and provided more electron-transfer tunnels between the CNT and graphene. This was especially observed when graphene was laid on top of the CNT film, as it completely wrapped the CNT film and/or flattened the raised large CNT bundles into the CNT film plane, thereby imparting a denser structure to Graphene/S-CNT/PET, which resulted in its lower resistance and higher transmittance compared with S-CNT/Graphene/PET. Further, the bending test and adhesion test with 3M tape suggested that the film heater had good mechanical and thermal stability. The portable heating test with 9 V battery and defrost test showed the possibility of the wide range of applications for the flexible and transparent hybrid 2D platform heater. In summary, the transparent, flexible, and lightweight Graphene/S-CNT/PET hybrid film heater demonstrated good stability and can replace the commercial ITO for application in vehicle defrosting, smart windows, portable heating, and mobile defogging as well as in other fields.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Raman spectra of CNT and graphene, temperature-voltage curves, FE-SEM images of hybrid devices, AFM dynamic statics, transmittance when using PET as baseline and temperature of Graphene/S-CNT/PET during 4 h working duration (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. -M. Kim) *E-mail: [email protected] (H. W. Lee)

Author Contributions The main experiments were conducted by L. Li, and the various properties measurements were conducted by S. K. Hong, Y. Jo, M. Tian, and C. Y. Woo. Prof. S. H. Kim analyzed the measurement results and Prof. J. –M. Kim and Prof. H. W. Lee organized all in this study and writing a manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This research was supported by Basic Science Research Program Through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07046551), and supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF-2017M3D1A1039287). Also, this work was supported by the National Research Foundation (NRF-2018R1A5A1025594) of the Ministry of Science and ICT.

ABBREVIATIONS S-CNT/PET, Single layer carbon nanotubes/PET; D-CNT/PET, Double layers carbon nanotubes/PET;

S-CNT/Graphene/PET,

Single

layer

carbon

nanotubes/Graphene/PET;

Graphene/S-CNT/PET, Graphene/Single layer carbon nanotubes/PET.


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Transparent, Flexible Heater Based on Hybrid 2D Platform of

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