Self-Healing Polymer Dielectric for a High Capacitance Gate Insulator

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Self-healing Polymer Dielectric for a High Capacitance Gate Insulator Jieun Ko, Young-Jae Kim, and Youn Sang Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08220 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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ACS Applied Materials & Interfaces

Self-healing Polymer Dielectric for a High Capacitance Gate Insulator Jieun Ko,†, # Young-Jae Kim,*,†,# and Youn Sang Kim*,†, ‡ †

Program in Nano Science and Technology, Graduate school of Convergence Science and

Technology, Seoul National University, Seoul, 08826, Republic of Korea ‡

Advanced Institutes of Convergence Technology, 145 Gwang gyo-ro, Yeongtong-gu,

Suwon-si, Gyeonggi-do, 16229, Republic of Korea KEYWORDS. Self-healing electronics; Gate insulator; High-capacitance; Flexible electronics; Thin-film Transistors

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ABSTRACT: Self-healing materials are required for the development of various flexible electronic devices to repair the cracks and ruptures caused by repetitive bending or folding. Especially, a self-healing dielectric layer has huge potential for achieving healing electronics without mechanical breakdown in flexible operations. Here, we developed a high performance self-healing dielectric layer with an ionic liquid and catechol-functionalized polymer, which exhibited a self-healing ability for both bulk and film states under mild selfhealing conditions at 55 °C for 30 min. Due to the sufficient ion mobility of the ionic liquid in the polymer matrix, it had a high capacitance value above 1 µF/cm2 at 20 Hz. Moreover, zinc oxide (ZnO) thin-film transistors with a self-healing dielectric layer exhibited a high field-effect mobility of 16.1 ± 3.07 cm2 V-1 s-1 at a gate bias of 3 V. Even after repetitive selfhealing of the dielectric layer from mechanical breaking, the electrical performances of TFTs were well maintained.


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1. INTRODUCTION In the last decade, self-healing electronics have been actively investigated for next generation devices with a self-healing ability.1-3 Especially for flexible and foldable devices, it is necessary to improve the reliability of these devices from cracks and breaks that occur during repetitive use and stress. However, most self-healing applications have been limited to a few components, such as coating films4,5 and conductive materials.6-9 Particularly, for self-healing electronics of conductive materials, bulk conductive electrodes6,7 and conductive wires8,9 have mainly been investigated in both single and integrated electrodes in devices. Selfhealing dielectrics, meanwhile, have advantages especially in TFTs as a crack-free gate insulator against external shocks. In general, the gate dielectric has the thickest layer among the components of the TFT. Therefore, the dielectric layer is subjected to large bending strain under repetitive mechanical deformation of the ultimate flexible TFT which is composed of all soft materials, because bending strain is proportional to the thickness of bent layer.10 Fracture or delamination at the interface of dielectric layer induced by bending strain could cause gate leakage currents and act as scattering sites of charge carrier which incur degradation of current density, resulting in critical failures in devices.11,12 In order to overcome the aforementioned problems, self-healing dielectrics have been developed with lipid monolayers,13 natural aluminum oxides,14 and polymer blends.15 However, most of the research has focused on the self-healing ability of electric breakdown. Additionally, they have limitations in practical applications due to a slow self-healing process. Therefore, robust and high performance self-healing dielectrics still remain an urgent challenge for practical applications.



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Herein, we introduce a self-healing high capacitance dielectric consisting of a catecholfunctionalized polymer and ionic liquid, which has excellent self-healing, stable and high performance device characteristics. The dielectric layer exhibited a good self-healing property attributed to hydrogen bonding between the catechol groups and ionic liquids. Moreover, due to the sufficient amount and high capacitance value of the ionic liquid in the catechol-functionalized polymer matrix, the self-healing polymer dielectric layer had a high capacitance value above ~1 µF/cm2 at 20 Hz. To characterize the self-healing polymer dielectric layer as a gate insulator, we fabricated top-gate and side-gate zinc oxide thin-film transistors (ZnO TFTs). These exhibited high electrical properties with a linear field-effect mobility of 16.1 ± 3.07 cm2 V-1 s-1 and 12.9 ± 1.59 cm2 V-1 s-1 at a gate bias of 3 V, respectively. Additionally, in combination with its self-healing ability and high capacitance, the self-healing polymer dielectric layer integrated side-gate ZnO TFTs exhibited remarkable self-healing behaviors and high electrical performance with a linear field-effect mobility of ~12 cm2 V-1 s-1 at a gate bias of 3 V even under repeated mechanical breaking and healing processes. 2. EXPERIMENTAL SECTION 2.1. Materials. 1-Ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide (EMITFSI), acetone, dimethylformamide, dopamine hydrochloride, methacrylate anhydride, methyl methacrylate, sodium tetraborate decahydrate and zinc oxide were purchased from Aldrich. Azobisisobutyronitrile (AIBN), ether, ethyl acetate, hexane, hydrogen chloride, magnesium sulfate, sodium bicarbonate, sodium hydroxide, and tetrahydrofuran were purchased from the Samchun Chemical Co. Ammonia solution (25%) was purchased from Wako. All organic solvents were used without any further purification. Synthesis of dopamine methacrylate (DMA) and poly(methylmethacrylate-co-dopamine methacrylate) 4 ACS Paragon Plus Environment

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(p(MMA-co-DMA)), and characterizations are provided in the Supporting Information. A red LED (BL-HS135A-TRB, Bright LED electronics corp.) was used for the LED test. 2.2. TFT Fabrication. The schematic illustration of TFT fabrication process is shown in Figure S1. The TFT device was fabricated on a heavily doped p-type Si (100) wafer with 200 nm-thick SiO2. The wafer was cleaned sequentially with acetone and isopropyl alcohol in an ultrasonic bath, and then dried with nitrogen and UVO treatment for 30 min. ZnO semiconductor was fabricated by following the previously reported method.16 First, 162.6 mg of ZnO is dissolved in 24 mL of NH4OH at 4 °C. This amine hydroxo zinc complex precursor contained solution was spin coated onto the wafer at 3000 rpm for 30 sec followed by thermal annealing at 300 °C for one hour. The Al source and drain electrodes were deposited on the ZnO layer by thermal evaporation with the channel length and width defined as 50 and 1000 µm, respectively. For self-healing dielectric layer, the p(MMA-co-DMA) and ionic-liquid (IL), EMI-TFSI, dissolved in acetone solution was prepared. The weight ratio of p(MMA-coDMA): IL: acetone was 1: 1.5: 7.5. The 100 µm thick-dielectric layer was prepared by solvent casting of the solution onto the rectangular-shaped PDMS (polydimethylsiloxane) cavity and then dried at 50 °C for 12 h. The PDMS cavity was used for protecting the contact area of source and drain electrodes. After removing the PDMS cavity, the top contact Ag electrode (100 nm) was finally deposited by thermal evaporation. Additionally, the side-gate TFT was fabricated with the same procedure. The channel length and width of the side-gate TFT was 150 and 3000 µm, respectively, and there was a gap between the gate electrode and the channel area with a 150 µm length. 10 devices were analyzed to obtain the average and standard deviation fabricated TFT. To characterize the flexible property of the device, sidegate flexible ZnO TFT was fabricated on a polyimide (PI) film. We used a metal oxide deposited PI film [Al2O3, 20 nm thick] to improve the interface conditions between the PI 5 ACS Paragon Plus Environment


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film and the ZnO layer. The ZnO layer was spin coated at 3000 rpm for 30 sec. And sequentially annealed at 90 °C for 15 min and 300 °C for 40 min. And then, the same procedure with the TFT on the silicon wafer was done as above. For the MIM device, the Ag electrode was deposited on the 100 µm thick-IL-p(MMA-co-DMA) coated on the heavily doped p-type Si (100) wafer. To investigate capacitance depending on distance, the gap between Ag electrodes was varied from 150 µm to 1000 µm. 3. RESULT AND DISCUSSION 3.1. Fabrication of the self-healing dielectric layer. A scheme of the self-healing dielectric layer is shown in Figure 1. Ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI), is incorporated into poly(methylmethacrylateco-dopamine methacrylate) (p(MMA-co-DMA)). It is well known that the catechol groups of dihydroxyphenylalamine (DOPA) have significant roles as a component of the adhesive footpads of mussel, which have a strong adhesive property enabling mussels to adhere to wet surfaces even in rough seas.17 Furthermore, the metal ion-catechol coordination,18-20 interfacial hydrogen bonding,21,22 and aromatic interaction21 between the catechol groups are attributed to the self-healing ability of catechol factionalized polymers. To provide the selfhealing ability of catechol to the dielectric layer, we synthesized a catechol-functionalized copolymer, p(MMA-co-DMA), by free radical polymerization of methylmethacrylate and dopamine methacrylate according to a modified method previously reported.23 After the polymerization, the vinyl groups in MMA and DMA were gone from the 1H-NMR spectrum of p(MMA-co-DMA) (Figure S2). From the NMR spectrum, the molar ratio of MMA to DMA in p(MMA-co-DMA) was 26 : 1. The synthesis and characterization of p(MMA-coDMA) are described in detail in the supplementary information. To fabricate the self-healing high capacitance gate insulator, an IL-p(MMA-co-DMA) film, which contains the ionic 6 ACS Paragon Plus Environment

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liquid and p(MMA-co-DMA), was fabricated by solvent-casting. The IL-p(MMA-co-DMA) solution was prepared by dissolving the ionic liquid and polymer in acetone. The weight ratio of IL to p(MMA-co-DMA) was 1.5 : 1. The ionic liquid was well mixed with p(MMA-coDMA), and a uniform IL-p(MMA-co-DMA) film was generated without phase separation. It was reported that ionic liquid in the PMMA matrix acts as a plasticizer and lowers the glass transition temperature (Tg) of the polymer.24-26 As shown in Figure S3a, the Tg of the PMMA and p(MMA-co-DMA) is 100.1 °C and 133.5 °C, respectively. After mixing with ionic liquid, the Tg of IL-PMMA and IL-p(MMA-co-DMA) decreased to 52.5 °C and 37.4 °C, respectively, due to the plasticizing effect of the ionic liquid (Figure S3b). 3.2. Self-healing characterization of the dielectric layer. To characterize the self-healing ability, a scratch was made with a razor blade on the PMMA, p(MMA-co-DMA), IL-PMMA and IL-p(MMA-co-DMA) film surfaces. And then, the films were placed on a 55 °C hot plate for 30 min. Due to the high Tg of PMMA and p(MMA-co-DMA), the damaged portion in the film was not repaired (Figure 2a). In the IL-p(MMA-co-DMA), the scratch had completely disappeared after 30 min at 55 °C (Figure 2a). At a temperature above the Tg of IL-p(MMAco-DMA), polymer chains gain mobility and can diffuse into the damaged area.27 As a result, bulk IL-p(MMA-co-DMA) also exhibited a self-healing ability when two pieces of the sample were put together and heated at 55 °C for 30 min (Figure 2b). On the other hand, the damaged area in the IL-PMMA film was partially repaired despite the decreased glass transition temperature by the ionic liquid (Figure 2a). Even after a few hours, the scratch on the IL-PMMA film was not fully mended. Therefore, we as-sumed that the catechol groups in the p(MMA-co-DMA) have an important role in the self-healing property. In the IL-p(MMAco-MMA), two possible hydrogen bonding interactions exist; one is the interaction between the ionic liquid and catechol groups, and the other is the catechol-catechol interaction (Figure 7 ACS Paragon Plus Environment


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1a). The ATR FT-IR spectra of DMA and IL-DMA are shown in Figure S4. The O-H stretching (νO-H) at the 3400~3200 cm-1 region was slightly shifted after the addition of the ionic liquid, which could be caused by the hydrogen bonding interaction between the ionic liquid and the hydroxyl groups in the DMA. A previous report showed that the ionic liquid could interact with the hydroxyl groups in a polymer through hydrogen bonding.28 In addition, the catechol can interact with neighboring catechols through hydrogen bonding.21,22 Therefore, we assumed that the self-healing is initiated by the diffusion of the polymer chains to the damaged area and accelerated by the interfacial hydrogen bonding of the catechol-ionic liquid and catechol-catechol groups. To investigate the effect of the catechol group on adhesion related to self-healing performance, force-distance curves of PMMA, p(MMA-co-DMA), IL-PMMA and ILp(MMA-co-DMA) were obtained by Scanning Probe Microscopy (Figure S5). From the Force-distance curve, the adhesion force of the four different samples was calculated using Hooke’s law, and the values for PMMA, p(MMA-co-DMA), IL-PMMA and IL-p(MMA-coDMA) were 0.14, 0.21, 0.36, and 1.66 nN, respectively. By introducing the catechol groups into the PMMA, the adhesion force of p(MMA-co-DMA) increased by about two times that of those PMMA (Figure 2c). It is well known that catechol has a high adhesion force to various surfaces by forming strong bidentate hydrogen bonding and metal coordination.17,29,30 After ionic liquid molecules were mixed with the polymer, the adhesive force increased to 0.36 and 1.66 nN for the IL-PMMA and IL-p(MMA-co-DMA), respectively. According to the literature, the ionic liquid can interact with the surface through van der Waals force or hydrogen bonding;31,32 thus, the adhesion force of IL-PMMA was increased compared to the PMMA. On the other hand, IL-p(MMA-co-DMA) has a much higher adhesion force than that of the other materials due to the synergic effect of the catechol group and ionic liquid. These 8 ACS Paragon Plus Environment

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results indicate that the catechol groups at the interface of the damaged area have an important role in the self-healing and the ionic liquids in polymer boost it. 3.3. Electrical characterization of the self-healing dielectric layer. To assess the possibility of IL-p(MMA-co-DMA) as a self-healable dielectric layer, the capacitance of ILp(MMA-co-DMA) was measured in the frequency range from 20 Hz to 10 kHz (Figure 3). The capacitance was obtained from the metal-insulator-metal (MIM) structure with a 100 µm thick IL-p(MMA-co-DMA) and from the side of the electrode structure for which the distance between the electrodes was 150, 300, 500, and 1000 µm, respectively (Figure 3a). As shown in Figure 3b, the IL-p(MMA-co-DMA) exhibited a high capacitance above ~1 µF/cm2 at 20 Hz and remained above 100 nF/cm2 at 10 kHz. In addition, the capacitance values for the various frequency ranges were maintained even though the gap between the two electrodes was increased from 150 to 1000 µm. The high capacitance value and distance independent capacitance of IL-p(MMA-co-DMA) were attributed to the high ion mobility of the ionic liquid molecules induced by the electric field and formation of electrical double layers (EDLs).33-35 The decreasing capacitance with increasing frequency is an intrinsic feature of the ionic liquid in the polymer dielectric, which is related to the speed of the ionic molecules depending on the frequency.34 The capacitance value of 1.63 µF/cm2 at 20 Hz was further used to calculate the mobility. Additionally, the thermal stability and self-healing ability of the dielectric layer was determined. We conducted a thermal stress of the MIM structure device at 120 °C for 1 h. As shown in Figure 3c, the capacitance value of the MIM device was well maintained after the thermal stress. Moreover, to characterize the capacitance of the recovered self-healing dielectric layer, the center of two electrodes on the capacitance device with a 150 µm gap was mechanically broken into two pieces by cutting the device with a diamond knife. For self-healing of the IL-p(MMA-co-DMA) dielectric layer, two 9 ACS Paragon Plus Environment


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pieces of the capacitance device were fused by heating at 55 °C on a hot plate for 30 min. As a result, the capacitance value for the 150 µm gap electrodes was maintained with similar capacitance values before and after the self-healing at frequency ranges from 20 Hz to 10 kHz (Figure 3d). These results show that the IL-p(MMA-co-DMA) dielectric layer has good thermal stability and a self-healing ability without degradation of the capacitance value. 3.4. Characterization of thin-film transistors with a self-healing dielectric layer. Figure 4a shows a schemat-ic illustration of the top-gate and top-contact ZnO TFT using the ILp(MMA-co-DMA) as a dielectric layer. Figure 4b shows the drain current–drain voltage (IDVD) output characteristics of the top-gate ZnO TFT as a function of the gate voltage (VG). The transfer characteristic was measured by sweeping VG from -1 to 3 V at a VD of 1 V (Figure 4c). The ZnO TFT had an on/off current ratio of ~105. The field-effect mobility (µ) of the ZnO TFT was calculated in the linear regime with the following equation (1): ‫ܫ‬஽ = ߤ‫ܥ‬

ௐ ௅

ቂሺܸீ − ܸ௧௛ ሻܸ஽ −

௏ವ మ ଶ

ቃ

(1)

, where W is the channel width; L is the channel length; C is the capacitance of the dielectric layer and Vth is the threshold voltage. From the equation, a field-effect mobility of 16.43 cm2 V-1 s-1 was obtained at VG = 3 V and VD = 1 V. The average of mobility, on/off current ratio and threshold voltage were 16.1 ± 3.07 cm2 V-1, 9.3 × 104 ± 0.1 × 104 and 0.27 ± 0.16 V, respectively. Although a small amount of hysteresis was observed in the transfer curve (Figure S6), the ZnO TFT had high electric performance at a low operating voltage as a result of the high capacitance of the IL-p(MMA-co-DMA). To demonstrate the self-healing property of the IL-p(MMA-co-DMA) dielectric layer in a device, a side-gate and top-contact ZnO TFT was fabricated (Figure 4d). In the side-gate ZnO-TFT configuration, the horizontal distance between the side-gate electrode on the IL10 ACS Paragon Plus Environment

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p(MMA-co-DMA) dielectric layer and the center of the ZnO channel was ~150 µm. From the output and transfer curve (Figure 4e-f), the side-gate ZnO TFT showed slightly degraded electrical properties compared with the top-gate ZnO TFT due to the mobility degradation effect in the side-gate TFT.32 However, it still exhibited a high electrical performance with a calculated on/off current ratio and carrier mobility of ~103 and 12.9 cm2 V-1 s-1 at VG = 3 V and VD = 1 V, respectively. The calculated average of mobility, on/off current ratio and threshold voltage of side gate TFTs were 12.9 ± 1.59 cm2 V-1 s-1, 2.9 × 103 ± 3.8 × 103 and 0.11 ± 0.10 V. Because charge accumulation at the dielectric/semiconductor interface is induced by the movement of ionic molecules in the IL-p(MMA-co-DMA) driven by the gate bias, TFT could not be operated when the dielectric layer between the channel and gate electrode was separated. To evaluate the self-healing property of the IL-p(MMA-co-DMA) dielectric layer, the gap between the channel and gate electrode of the side-gate ZnO TFT was cut into two pieces to represent a damaged gate insulator only. And then, the two pieces of the side-gate TFT were attached together, followed by heating at 55 °C for 30 min (Figure 5a). Optical microscope images and a photograph indicate that the IL-p(MMA-co-DMA) dielectric layer completely healed after the cutting and healing process of the side-gate ZnO TFT device (Figure 5b, Figure S7). Even the small scratches, which inadvertently were made on the dielectric surface during the cutting, also disappeared through the self-healing property. In addition, two pieces of IL-p(MMA-co-DMA) film on a transparent glass substrate were integrated into one piece after the same healing process (Inset of Figure 5b). The film thickness was the same as that of the dielectric layer in the side-gate ZnO TFT. This result confirms that the IL-p(MMA-co-DMA) dielectric layer in the side-gate ZnO TFT completely healed. Figure 5c shows the transfer curve of the side-gate ZnO TFT with the structure shown 11 ACS Paragon Plus Environment


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in Figure S7. The electric performance of the side-gate ZnO TFT still remained even after repeated cutting and healing processes, which can be attributed to the retained ionic mobility of the IL-p(MMA-co-DMA). After healing the dielectric layer of the side-gate ZnO TFT, it had an on/off ratio of ~104 and a mobility of 12.17 cm2 V-1 s-1 at VG = 3 V and VD = 1 V. Remarkably, the side-gate ZnO TFT still showed excellent electrical performance at a low operating voltage during the repeated cutting and healing test (Figure 5c-f), and the on/off current ratio and the mobility remained around ~104 and ~12 cm2 V-1 s-1, respectively (Table 1). These results demonstrate the stability of the IL-p(MMA-co-DMA) dielectric layer under repetitive cutting and healing processes at the same position. To demonstrate the healing property of the self-healing gate insulator, a red light-emitting diode (LED) lamp was connected in series with the side-gate TFT shown in Figure 6. The red LED lamp was lit up by the drain current of the side-gate TFT. After breaking the gap between the channel and the gate electrode, the red LED could not be turned on. The red LED was lit up again after the self-healing process of the gate insulator in the side-gate TFT (see Movie S1). Therefore, the results show the high self-healing performance of the IL-p(MMA-co-DMA) dielectric layer as a self-healing gate insulator for TFTs. Furthermore, the IL-p(MMA-co-DMA) was exploited as a dielectric layer in a flexible sidegate ZnO TFT on a polyimide substrate (Figure S8a). The transfer characteristics of the flexible side-gate ZnO TFT are shown in Figure S8c; the on/off current ratio was ~103, and the mobility was calculated to be 1.2 cm2 V-1 s-1 at VG = 3 V and VD = 1 V before the bending test. To evaluate the mechanical flexibility and electrical stability of the flexible side-gate ZnO TFT on polyimide in the bending state, the device was subjected to bending strain (Figure S8b). The flexible side-gate ZnO TFT still operated well under a bending radius of 10 mm (Figure S8c-e). The on/off current ratio and the mobility of the flexible ZnO TFT in the 12 ACS Paragon Plus Environment

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bending state were 104 and 1.2 cm2 V-1 s-1, respectively. These results indicate that the ZnOTFTs with IL-p(MMA-co-DMA) on a flexible polyimide substrate have mechanical flexibility and electrical stability under bending strain. In addition, we expect that the excellent adhesion of the IL-p(MMA-co-DMA) will be helpful in preventing device failures caused by delamination which generally is induced by repeated bending of a flexible device.

4. CONCLUSION In this study, we successfully prepared a self-healable dielectric material which consists of an ionic liquid and catechol-functionalized copolymer, IL-p(MMA-co-DMA). The damaged ILp(MMA-co-DMA) is completely repaired under mild conditions because of the low glass transition temperature from the plasticizer effect of the ionic liquid and the interfacial hydrogen bonding interaction related to the catechol groups. The presence of catechol groups was found to have an important role in the self-healing phenomenon of IL-p(MMA-co-DMA) when compared with IL-PMMA.

IL-p(MMA-co-DMA) had a high capacitance above ~1

µF/cm2 at 20 Hz, and the capacitance values were independent of the distance between the electrodes because of the high ionic mobility. When the IL-p(MMA-co-DMA) was applied to the ZnO TFT as a dielectric layer, the device still showed high electrical performance. The top-gate ZnO TFT had a high field-effect mobility of 16.1 ± 3.07 cm2 V-1 s-1 at a 3 V operating voltage and the side-gate ZnO TFT exhibited a mobility above ~12 cm2 V-1 s-1 at 3 V even after repeated cutting and healing tests. In addition, we fabricated a flexible ZnO TFT with the IL-p(MMA-co-DMA) dielectric on a polyimide substrate which stably operated under a severe bending strain. We expect that the IL-p(MMA-co-DMA) dielectric with its self-healing property has good potential for use in various self-healable electronic devices. 13 ACS Paragon Plus Environment


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Figure 1. (a) Chemical structures of materials and schematic illustration of IL-p(MMA-coDMA). (b) Illustration of self-healing process.


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Figure 2. (a) Optical images of before and after the healing process of PMMA, p(MMA-coDMA), IL-PMMA and IL-p(MMA-co-DMA). Healing process was done by heating the polymer films on the Si (100) wafer for 30 min at 55 °C. (b) Demonstration of the selfhealing properties of the bulk IL-p(MMA-co-DMA); IL-p(MMA-co-DMA) was cut into two pieces and attached together followed by heating at 55 °C for 30 min for self-healing. ILp(MMA-co-DMA) was prepared with rhodamine B for observation. (c) Adhesion force of the polymer and IL-polymer.



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Figure 3. (a) Schematic illustration of the metal-insulator-metal (MIM) and side electrode structure and (b) capacitance value from the MIM and side electrode structure as a function of the electrode distances at different frequencies. (c) Capacitance value from the MIM structure before and after thermal stress under 120 °C for 1 h. (d) Capacitance value with the side-electrode structure with 150 µm gap before and after healing from mechanical breakdown.


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Figure 4. Schematics of (a) top-gate and (d) side-gate ZnO TFT. (b) and (e) Output and (c) and (f) transfer characteristics of the top-gate and side-gate ZnO TFT.



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Figure 5. (a) Schematic illustration of the self-healing process of ZnO-TFT with the ILp(MMA-co-DMA) dielectric layer: ZnO-TFT was cut between the gate electrode and sourcedrain electrode, and two pieces were attached together, followed by heating at 55°C for 30 min. (b) Optical microscope images of before and after the healing process. The insets show the self-healing process of the IL-p(MMA-co-DMA) on a glass substrate. (c) Transfer characteristics of the ZnO TFT after repetitive healing tests. Output characteristics of the healed ZnO TFT after (d) first, (e) second, and (f) third healing experiments.


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Figure 6. Schematic illustration and photographs of a red LED connected with the side-gate TFT in series. (a) The red LED is connected with the side-gate TFT by a gold wire and turned on by the drain current of the side-gate TFT. (b) After breaking the gap between the S-D channel and the gate electrode, the red LED could not be turned on. (c) After the healing process by the gate insulator of the side-gate TFT at 55 °C for 30 min, the red LED could be lit up again.



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Table 1. Electrical properties of the ZnO TFTs with IL-p(MMA-co-DMA) as a dielectric layer. Mobility

On/off current ratio

(cm2 V-1 s-1)

Vth (V)

Top-gate structured TFT

16.1

9.3×104

0.27

Side-gate structured TFT

12.9

2.9×103

0.11

1st healed TFT

12.2

1.8×104

0.25

2nd healed TFT

12.5

1.8×104

0.26

3rd healed TFT

12.3

7.0×104

0.36


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: . Additional experimental details, materials characterization data (1H-NMR, DSC, FT-IR, and Force-distance curve), table of capacitance values, hysteresis of the top-gate ZnO TFT, photograph of the healed ZnO TFT, and characterization of the flexible ZnO TFT. Movie of self-healing process of the TFT with LED shown in Figure 6.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions #

J. Ko and Y.-J. Kim contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 21 ACS Paragon Plus Environment


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This work was supported by the Center for Advanced Soft Electronics as Global Frontier Research Program (2013M3A6A5073177) of the Ministry of Science, ICT and Future Planning of Korea. The authors gratefully thank J.–A. Song and Prof. T.-L. Choi for the GPC measurements, and Dr. J. A. Lim for the valuable comments about the LED application

ABBREVIATIONS TFT

thin-film transistor

ZnO

zinc oxide

EMI-TFSI

1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

AIBN

azobisisobutyronitrile

PDMS

polydimethylsiloxane

IL

ionic liquid

p(MMA-co-DMA) DOPA Tg

dihydroxyphenylalamine glass transition temperature

MIM

metal-insulator-metal

ID

drain current

VD

drain voltage

VG

gate voltage

µ

poly(methylmethacrylate-co-dopamine methacrylate)

mobility 22 ACS Paragon Plus Environment

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W

channel width

L

channel length

C

capacitance

Vth LED

threshold voltage light-emitting diode

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Table of Contents Graphic and Synopsis


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We demonstrated a self-healing dielectric layer. By using the mixture of ionic liquid and catecholfunctionalized polymer as a dielectric layer, it showed a high capacitance value above 1 µF/cm^2 and good self-healing property. Additionally, the side-gate TFTs with self-healing gate insulator exhibited excellent electrical properties even after repeated self-healing tests with mobility of ~12 cm^2 V^-1 s^-1. 125x65mm (220 x 220 DPI)

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Self-Healing Polymer Dielectric for a High Capacitance Gate Insulator

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