Copper Grid Composites for High-Performance, Ultra-Stable

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Ionogel/Copper Grid Composites for High-Performance, Ultra-Stable Flexible Transparent Electrodes Li Chang, Xiqi Zhang, Yi Ding, Hongliang Liu, Mingzhu Liu, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09023 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Applied Materials & Interfaces

Ionogel/Copper Grid Composites Performance, Ultra-Stable Flexible Electrodes

for HighTransparent

Li Changa, Xiqi Zhangb, Yi Dingc,d, Hongliang Liub*, Mingzhu Liua* and Lei Jiangb* a

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal

Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou University, Lanzhou 730000, P. R. China b

CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of

Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China c

Beijing National Laboratory of Molecular Sciences (BNLMS), Key Laboratory of Organic

Solid, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China d

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Abstract Production of high-performance and stable low-cost copper (Cu)-based flexible transparent electrodes (FTEs) is urgently needed for the development of new-generation flexible optoelectronic devices, but it still remains challenging. Herein, we developed a facile approach to fabricate high-performance, ultra-stable Cu grid (CuG)-based FTEs by UV lithography-assisted electroless deposition of patterned Cu on flexible polyethylene terephthalate (PET), which is then

encapsulated

by

a

thin

poly(1-vinyl-3-ethylimidazolium

bis(trifluoromethanesulfonyl)imide) (P[VEIM][NTf2]) ionogel layer to improve the mechanical flexibility and stability. The as-prepared composite FTE (ionogel/CuG@PET) exhibits sheet

ACS Paragon Plus Environment

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resistance of 10.9 Ω sq-1 and optical transmittance of 90% at 550 nm. Introduction of the thin uniform P[VEIM][NTf2] ionogel nanofilm by virtue of the superwettability of the Cu layer endows the electrode with excellent mechanical flexibility and stability. This new highperformance Cu-based FTE should be attractive alternative to indium tin oxide (ITO) for practical optoelectrical applications.

Keywords: flexible transparent electrode, superwettability, copper grid, ionogel, and ionic liquid

The design of high-performance and cheap flexible transparent electrodes (FTEs) with characteristics of optical transparency, electrical conductivity and mechanical and long-term stability, plays an essential role in the development of next-generation flexible optoelectronic devices, including organic photovoltaics, organic light-emitting diodes and touch screen panels.17

In recent years, silver (Ag)-based FTEs in the forms of nanopatterns,8-9 nanowires10-13 or

continuous thin films14-21 have gained great progress, which exhibit superior optoelectrical properties for flexible applications compared to commercial indium tin oxide (ITO), a mechanically fragile material most commonly used in the transparent electrodes industry, and far superior optoelectrical properties to conducting polymers,22-25 carbon nanotubes26-28 and


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graphene1, 29-30-based FTEs. However, copper (Cu) with high electrical conductivity comparable to that of Ag, but low price of about 100 times less than that of Ag would be better as a promising candidate for replacing Ag within FTEs.31-32 Currently, random Cu nanowires (CuNWs) networks with high aspect ratios, relying on sophisticated solution processing, have achieved the same level of optoelectrical properties compared with Ag-based FTEs.31, 33-34 But CuNWs networks-based FTEs usually require additional heat treatments to reduce the contact resistance between CuNWs, which may destroy the flexible substrates.35 More seriously, CuNWs suffer from severe oxidation even under ambient conditions, resulting in a sharp increase in sheet resistance (Rsh) and haziness of the CuNWs-based FTEs, which hinders their use in practical applications. Efforts to prepare oxidation-resistant CuNWs have been made including surface passivation of the nanowires with inert inorganic coatings,36-39 surface-embedded CuNWs in organic polymer coatings40 or graphene.41-43 However, these strategies suffer drawbacks of decreased optoelectrical performance or complex experimental processes. Cu grids (CuGs) with continuous Cu pattern and space, should be excellent alternatives to fabricate high-performance FTEs because Rsh of the continuous Cu pattern is very low and the space can effectively improve its optical transmission. There have been a few attempts using nanosecond laser ablation,44 microsphere lithography,45 breath-figure polymer films template46 or UV lithography combining with subsequent wet etching47 to fabricate CuGs-based FTEs, but all of these strategies need a complicated Cu sputtering process and performance of these CuGs-based FTEs is not good enough for practical application. Therefore, it is still a big challenge to gain CuGs-based FTEs with combined excellent optoelectrical properties, mechanical stability and long-term stability, to satisfy the needs in the development of new-generation flexible optoelectronic devices.


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Results and Discussion Herein, we report a facile approach to construct a new type of composite electrode comprising layered Cu and ionogel nanofilm that shows outstanding mechanical and long-term stability with both high optical transparency and low Rsh. The designing concept is depicted in Figure 1a. First, patterned photoresist on flexible polyethylene terephthalate (PET) constructed by UV lithography was dipped in AgNO3 solution to deposite Ag seeds. Second, electroless deposition of Cu was performed by using Ag seeds as the catalyst. The CuG pattern on PET (CuG@PET) was easily achieved by subsequent photoresist lift-off. Scanning electron microscope (SEM) and corresponding energy dispersive spectrometer (EDS) mapping images show that the patterned CuG is well formed on PET (Figure 1b). Significantly, this method to construct CuG is very convenient because it avoids using the complicated sputtering process, a commonly used process to fabricate Cu thin film in constructing CuG.44-47 Moreover, the size of CuG@PET obtained using this method is accurate and adjustable which results in easily controllable optoeletrical performance. Third, an ionic liquid (IL) monomer (i.e., 1-vinyl-3ethylimidazolium bis(trifluoromethanesulfonyl)imide ([VEIM][NTf2])) together with an initiator and a cross-linker was introduced onto the CuG via complexation between Cu and the IL monomer to further enhance the mechanical and long-term stability of the CuG@PET. A thin uniform ionogel layer was formed on top of the superwettable Cu layer by subsequent in situ cross-linked polymerization of [VEIM][NTf2]. Thus, a new type of composite electrode consisting CuG covered with an ionogel nanofilm was generated (ionogel/CuG@PET). The successful introduction of the ionogel film is demonstrated by the presence of C, O, N, F and S elements, which belong to the P[VEIM][NTf2] ionogel (Figure 1c, Figure S1). SEM images


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further reveal that thickness of the Cu layer is 100~150 nm (Figure 1d and S2) and the covered thin ionogel film is 60~90 nm (Figure 1d). Transmission electron microscope (TEM) images of the dispersed ionogel/Cu composite peeled off from the PET substrate further demonstrate that the Cu layer is covered with a thin ionogel film (Figure 1e, f). The as-prepared ionogel/CuG@PET showing outstanding optoelectrical properties (Rsh of 10.9 Ω sq-1 and optical transmittance of 90% at 550 nm (T550)) is highly stable under ambient condition and can endure high temperature and humidity (85 oC/85% RH) for at least 45 hours. The thin uniform P[VEIM][NTf2] ionogel nanofilm by virtue of the superwettable property of the Cu layer can greatly improve mechanical flexibility and stability of the electrode. Owing to the facile fabrication process which does not depend on complex equipments and harsh conditions, we can easily fabricate an ionogel/CuG@PET composite electrode covering an area of 25 cm2 by using this approach (Figure 1g). In our design, the uniform ionogel nanofilm on top of the Cu layer is significantly important to obtain high-performance ionogel/CuG@PET composite FTEs. Our previous study has indicated that wettability regulation had important impact on the improvement of electrode performance.48 During this experiment, we found that wettability of the Cu layer by the IL monomer is critical for the formation of uniform ionogel nanofilm. We carefully regulate the morphologies of the deposited Cu layers to tune the surface wettability by the IL monomer, i.e., [VEIM][NTf2]. As shown in Figure 2a, when the deposition time of Cu is shorter than 2 min (Figure 2a i, ii), the deposited Cu layers are sparse and discontinuous, and cannot be well wetted by [VEIM][NTf2] with [VEIM][NTf2] contact angles (CAs) larger than 20o owing to the larger CA of uncovered PET substrates (Figure S3). Such large CAs lead to obvious discontinuous ILs


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islands on top of the Cu layers (Figure 2b i, ii, Figure 2c i, ii), which are unfavorable for the subsequent formation of uniform ionogel films. Moreover, the discontinuous Cu layers result in

Figure 1. Fabrication of ionogel/CuG@PET composite FTEs. (a) Schematic of the designing concept for ionogel/CuG@PET composite FTEs. UV lithography-assisted photoresist grid pattern on PET is dipped in AgNO3 aqueous solution to deposite Ag seeds. Then, CuG@PET is fabricated by electroless deposition of Cu with the Ag seeds serving as the catalyst. Subsequently, the CuG@PET is encapsulated by a thin ionogel nanofilm by virtue of superwettable property of the Cu layer. SEM and the corresponding EDS mapping images of the CuG@PET (b) and ionogel/CuG@PET (c). (d) A cross-section of the ionogel/CuG@PET shows


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that the Cu layer with thickness of 100~150 nm is covered with a thin ionogel film of 60~90 nm. (e) TEM image of the ionogel/Cu composite peeled off from the PET substrate. (f) Highresolution TEM image shows typical lattice distance of 0.21 nm belonging to the (111) plane of the Cu. (g) Photograph of a ionogel/CuG@PET film covering an area of 25 cm2. notably high Rsh, too high to detect by the four-point probe resistivity measurement system (Figure S4). When we increase the deposition time to 3 min, the generated Cu layer becomes denser with CA close to 10o (Figure 2a iii), which is defined as the CA boundary of superILphilicity49. The superILphilic character of the Cu layer facilitates the formation of a continuous IL film (Figure 2b iii, 2c iii). Further increasing the deposition time to 4 min, the Cu layer gets much denser with [VEIM][NTf2] CA down to about 4.4o (Figure 2a iv). However, the Cu layer with deposition time of 5 min is not so easy to be wetted in comparison to that with deposition time of 4 min. The slight increase of [VEIM][NTf2] CA (Figure 2a v) is probably owing to the decreased surface roughness (Figure 2a v). Considering both the IL wettability (Figure 2a) and conductivity (Figure S4) of the Cu layers, we choose deposition time of 4 min as the optimal condition for further study. The stability of the ionogel/CuG@PET composites strongly relies on the complexations between Cu and the ionogels, which are closely related to the chemical structures of the ionogels. Herein, we choose four types of commercial ionogels with different anions (Figure 3a) to evaluate conductivity of the composite electrodes. For the composite electrodes with P[VEIM][NTf2] and poly(1-vinyl-3-ethylimidazolium tetrafluoroborate) (P[VEIM][BF4]) as the ionogel nanofilms, Rsh of the obtained electrodes is about 1.8 Ω sq-1 and 2.1 Ω sq-1 respectively, comparable to the Cu layer on PET (Cu@PET) without ionogel (about 1.1 Ω sq-1) (Figure S5). Optical microscope images show that morphologies of the Cu layers have no obvious changes


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after introduction of P[VEIM][NTf2] and P[VEIM][BF4] ionogels (Figure 3b i, ii). While for composite electrodes with poly(1-vinyl-3-ethylimidazolium bromide) (P[VEIM]Br) and poly(1-

Figure 2. Regulating IL wettability to form uniform ionogel nanofilms on the Cu@PET. (a) SEM images of Cu@PET show that the Cu layers change from sparse and discontinuous to dense and continuous with increasing deposition time, 1 min (i), 2 min (ii), 3 min (iii), 4 min (iv) and 5 min (v). The inserts show the corresponding IL CAs. SEM (b) and fluorescent microscope images (c) show that discontinuous IL islands are formed on the Cu@PET with deposition time of 1 min (i) and 2 min (ii), and continuous IL films are generated on the Cu@PET with deposition time of 3 min (iii), 4 min (iv) and 5 min (v). The [VEIM][NTf2] was dyed red with Nile red. vinyl-3-ethylimidazolium dicyanamide) (P[VEIM][DCA]) as the ionogel nanofilms, the Cu layers are destroyed to a large extent after introduction of the ionogels, and thus Rsh of the electrodes increases by five orders of magnitude (Figure 3b iii, iv). To understand the influence of different ionogels on conductivity of the electrodes at the molecular level, we used a quartz


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crystal microbalance (QCM) to investigate the molecular interactions between Cu and corresponding IL monomers. Figure 3c depicts typical sensorgrams upon injecting of various IL monomers (5 mM) to Cu-coated QCM sensors. After injection of 5 mM ethanol solution of [VEIM][NTf2], frequency changes decrease immediately, indicating formation of a stable complex between Cu and [VEIM][NTf2] (Figure 3c i, d i). The binding constant (Ka) determined by QCM measurements is 29.2 M-1 (Figure S6). When injecting 5 mM ethanol solution of [VEIM][BF4], frequency changes are slightly increased by 1 Hz and then keep constant, suggesting that Cu has neglectable interaction with [VEIM][BF4] (Figure 3c ii, d ii). While for [VEIM]Br, introduction of [VEIM]Br leads to sustained increase of frequency changes, which reveals that Cu can react with [VEIM]Br and then dissolve in ethanol (Figure 3c iii, d iii). This phenomenon was consistent well with the results that the Cu layer is severely damaged by [VEIM]Br (Figure 3b iii). We also found an interesting phenomenon when 5 mM aqueous solution of [VEIM][DCA] contacted with the Cu-coated QCM sensors. The frequency changes increase first and then decrease rapidly (Figure 3c iv). The increased frequency changes attribute to severe corrosion of Cu by [VEIM][DCA] just as the situation of [VEIM]Br. However, the formed complexation between Cu and [VEIM][DCA] cannot be dissolved in the aqueous solution (Figure 3d iv, S7), but deposits on the Cu-coated QCM sensor, making the frequency changes decrease. Therefore, we choose [VEIM][NTf2] that can form stable complexation with Cu to fabricate high-performance composite electrodes. Optoelectrical properties (i.e., Rsh and T550) are among the most important parameters for FTEs, and were thus optimized in detail. Rsh and T550 of the CuG@PET in our work can be easily controlled by adjusting the size of the photoresist mask. As shown in Figure 4a-c, we have fabricated a series of CuG@PET with varied pattern line width (W) and line space (S). The line


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thickness and width of CuG@PET were further confirmed by atomic force microscopy (AFM) (Figure S8). When W is fixed to 10 µm, Rsh of the CuG@PET is gradually increased from about 0.8 Ω sq-1 to about 2.2 Ω sq-1 with increasing S from 50 µm to 400 µm (Figure 4d), which is probably owing to the decreased density of the micro-scaled CuG. This decreased density of the micro-scaled CuG also leads to increased T550 (Figure 4d). The same phenomenon for Rsh and T550 can also be achieved for the CuG@PET with W of 5 and 2 µm (Figure 4e, f). Significantly, it shows that all the CuG@PET with W of 10 µm, 5 µm and 2 µm can exhibit excellent optoelectrical property for high-performance FTEs. However, to avoid moire effect,50-51 we choose W = 2 µm to produce ionogel/CuG@PET composite electrode for further study.


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Figure 3. Influence of chemical structures of ionogels on performance of the composite electrodes. (a) Chemical structures of ionogels used in this study. (b) Optical microscope images show that morphologies of the composite electrodes with P[VEIM][NTf2] and P[VEIM][BF4] are uniform and continuous with Rsh comparable to Cu@PET. While morphologies of the composite electrodes with P[VEIM]Br and P[VEIM][DCA] are severely destroyed with an increment of Rsh by five orders of magnitude. (c) Typical curves of frequency changes following injection of 5 mM IL solution into the QCM chamber with Cu-coated QCM sensors. (d) Corresponding cartoons depict the interactions between the Cu-coated sensors and different ILs.

Figure 4. Optimizing optoelectrical properties of the CuG@PET. SEM images of CuG@PET with W of 10 µm and increasing S from 50 µm to 400 µm (a), W of 5 µm and increasing S from 25 µm to 200 µm (b) and W of 2 µm and increasing S from 10 µm to 80 µm (c). Plots of Rsh and T550 against S for CuG@PET films of 10 µm (d), 5 µm (e) and 2 µm (f) show that Rsh and T550 of the CuG@PET increase gradually with increasing S.


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We systematically investigate the optoelectrical properties, flexibility, mechanical and stability of the electrodes without (CuG@PET) and with P[VEIM][NTf2] ionogel (ionogel/CuG@PET). It shows that introduction of the P[VEIM][NTf2] ionogel only slightly increases the Rsh (Figure 5a), while keep the optical transmittance almost unchanged (Figure 5b). To balance the Rsh and T550 with the desire for practical applications, we choose W = 2 µm and S = 40 µm as the optimal condition. The corresponding ionogel/CuG@PET possesses Rsh of 10.9 Ω sq-1 and T550 of 90%, which exhibits better optoelectrical property than most of the Cu-based transparent electrodes33-37, 39-46, 52-60 except Cu nanotrough (Cu NT)61 and Cu vein62 (Figure 5c). Significantly, ionogel/CuG@PET exhibits outstanding flexibility compared with CuG@PET, which benefits from the flexible P[VEIM][NTf2] ionogel layer facilitating the stable existence of the conductive CuG during mechanical deformation. The bending tests at different radii suggest that Rsh of the ionogel/CuG@PET almost has no significant change while Rsh of CuG@PET gradually increases at radii ranging from 30 cm to 5 cm (Figure S9). Moreover, Rsh of the ionogel/CuG@PET can keep constant even after 2000 bending cycles (Figure 5d). In contrast, Rsh of CuG@PET continues to increase and cannot be detected after 1600 bending cycles (Figure 5d). We further used tape tests to explore the mechanical stability of the complex ionogel/CuG@PET. After 30 times tape tests by using a 3M Scotch® tape, Rsh of the ionogel/CuG@PET just has a slight increase (Figure 5e), and no CuG can be observed on the tape (Figure S10a). In contrast, Rsh of the CuG@PET is greatly increased during the tape tests, 22-times higher after 12 times tape tests (Figure 5e), with obvious CuG adhered on the tape (Figure S10b). Moreover, both the CuG@PET and ionogel/CuG@PET fabricated by using this method are air-stable, with Rsh unchanged after storage under ambient conditions for at least 60 days (Figure S11). More importantly, the ionogel/CuG@PET also shows excellent air-stability


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under harsh environment. Rsh remains at its initial value for at least 45 hours even at high temperature and humidity (85 oC/85% RH), while Rsh of CuG@PET is about 300 times higher than its initial value after 14 hours under the same condition (Figure 5f). We attribute this outstanding performance of the ionogel/CuG@PET to the introduction of the thin stable ionogel

Figure 5. Performance of our designed CuG@PET and ionogel/CuG@PET. (a) Introduction of the ionogel nanofilm leads to only slight increase of the Rsh, while keep the optical transmittance


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almost unchanged (b). (c) Plots of T550 versus Rsh for our designed ionogel/CuG@PET with previously reported Cu-based electrodes including CuNW,33-35,

40, 52-56

graphene/CuNW

(G/CuNW),41-43 Ni/CuNW,36-37 AZO/CuNW,39, 57-58 Cu NT,61 Cu vein,62 Cu grid,44-46 Cu alloy grid59 and Cu thin film60. The ionogel/CuG@PET composite film exhibits much better flexibility (d) and mechanical stability (e) than CuG@PET. The bending radius in (d) is 10 cm. The insert in (e) shows the schematic of the tape tests (using high-tack 3M Scotch® tape). The tape was first pressed with a finger and then carefully peeled off. (f) The ionogel/CuG@PET shows excellent air-stability at high temperature and humidity (85 oC/85% RH) for at least 45 hours while a significant increase of Rsh appears and cannot be detected after 14 hours for CuG@PET under the same condition. The above results were all obtained using the CuG@PET and ionogel/CuG@PET with W of 2 µm. film, which is hydrophobic and dense enough and thus can efficiently protect Cu from moisture and oxygen. This unique property can solve the problem that Cu-based electrodes tend to be oxidized during the process of production, transportation and storage, and can thus avoid the usage of special conditions such as inert atmosphere and further reduction process. As a demonstration, we have successfully applied our designed composite electrodes for a touch screen panel (Figure S12). In addition, this ionogel/CuG@PET can be further prepared in large scale by using soft lithography to construct large-scale patterned Ag seed layer (Figure S13). Conclusion In conclusion, we have developed a facile approach to fabricate ultra-stable Cu-based FTEs through UV lithography-assisted electroless deposition of patterned Cu, which is then encapsulated by a thin P[VEIM][NTf2] ionogel nanofilm. The designed ionogel/CuG@PET


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composite electrode exhibits Rsh of 10.9 Ω sq-1 and T550 of 90%. The ionogel/CuG@PET fabricated by this approach is highly stable under ambient conditions and even at high temperature and humidity. More importantly, introduction of a thin P[VEIM][NTf2] ionogel nanofilm also endows the electrode with excellent flexibility and mechanical stability. By addressing the long-term and mechanical stability simultaneously, our Cu-containing electrodes should be attractive alternatives to large-scale and low-cost FTEs.

Materials and Methods Materials: All materials were obtained from the suppliers as follows and were used as received. SPR-220-7 photoresist was purchased from Suzhou Ruicai Semiconductor Co., Ltd. Silver nitrate (AgNO3, AR) was purchased from Guangdong Guanghua Technology Co., Ltd. N,N’-Methylenebis(acrylamide) (BIS, 99%), Cupric chloride dihydrate (CuCl2·2H2O, AR), Ethylenediaminetetraacetic acid disodium salt dehydrate (Na2EDTA·2H2O, AR, 98%) and Potassium sodium tartrate tetrahydrate (NaKC4H4C6·4H2O) were purchased from SigmaAldrich. Formaldehyde (HCHO, 37~40%, AR) was provided by Xilong Chemical Co., Ltd. Glycol dimethacrylate (EGDMA, 99%), Nile red (97.5%) and 2,2’-Azobisisobutyronitrile (AIBN, 99%) were obtained by J&K Scientific. Sodium hydroxide (NaOH, AR), Ammonium persulphate (APS, AR), Ethanol (AR), Acetone (AR) and Ethylene glycol (AR) were supplied by Beijing Chemical Works. 1-Vinyl-3-ethylimidazolium dicyanamide ([VEIM][DCA], >99%), 1Vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM][BF4], >99%), 1-Vinyl-3-ethylimidazolium bromide

([VEIM]Br],

>99%)

and


1-Vinyl-3-ethylimidazolium

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bis(trifluoromethanesulfonyl)imide ([VEIM][NTf2], >99%) were purchased from Lanzhou Yulu Fine Chemical Co., Ltd. Fabrication of CuG@PET: The CuG@PET was fabricated via four primary steps: constructing micro patterns, deposition of Ag seeds, chemical deposition of Cu and photoresist lift-off. SPR-220-7 photoresist was spun coat onto PET and then baked at 115°C for 180 seconds in air. A patterned photoresist on PET was obtained via subsequent UV lithography. The patterned photoresist on PET was immersed in AgNO3 aqueous solution to deposite Ag seeds and subsequently immersed into the Cu electroless deposition solution for one to five minutes to deposite Cu. The aqueous solution for Cu electroless deposition contains 12 g L-1 CuCl2, 14 g L-1 NaOH, 25 g L-1 Na2EDTA·2H2O, 14 g L-1 NaKC4H4C6 and 2.2 g L-1 HCHO. The CuG@PET was obtained by removing photoresist with acetone before dried with a flow of nitrogen. Fabrication of ionogel/CuG@PET: The ionogel/CuG@PET was prepared by spin coating of IL prepolymer on the surface of CuG@PET and subsequently polymerization at 80 °C for 1 h under N2 atmosphere. For P[VEIM][DCA], P[VEIM][BF4] and P[VEIM]Br, the prepolymer is composed of IL monomer as precursor, N,N’-methylenebis(acrylamide) as cross-linker and ammonium persulphate as initiator. While for P[VEIM][NTf2], glycol dimethacrylate and 2,2’azobisisobutyronitrile were used as cross-linker and initiator, respectively. Fabrication of Cu@PET with and without ionogel: The Cu@PET with and without ionogel were prepared in a similar manner as ionogel/CuG@PET and CuG@PET, except that uniform Ag seeds on PET prepared by dropping solution of 3% AgNO3 on PET was used. QCM Measurements: All QCM measurements were performed at 25 °C using Q-Sense E1 system (Sweden). The QCM channel was washed with deionized water or ethanol before binding


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assays between Cu substrates and ILs. The ILs were diluted with ethanol for [VEIM][NTf2], [VEIM][BF4] and [VEIM]Br, and deionized water for [VEIM][DCA]. The solution was injected into the channel at a flow rate of 100 µL min−1 to obtain the binding curves. All binding curves were recorded by Q-Sense software and analyzed by QTools. Characterization: The structures and morphologies of the CuG@PET, ionogel/CuG@PET and Cu@PET with and without ionogel were investigated by a Hitachi S-4800 scanning electron microscope, an Olympus BX51 optical microscope and a JEM-2100F transmission electron microscope. The transmittance of the FTEs was measured using an ultraviolet–visible spectrophotometer (UV-2600, SHIMADZU) in the wavelength range of 400–800 nm. The baseline on the ultraviolet–visible spectrophotometer was corrected using a clear PET substrate. The mechanical flexibility of the films as function of number of bend cycles was also determined by repeatedly bending the composite films with a two-point bending device (LTS150/M, THORLABS). The tape tests were conducted by applying 3M Scotch® tape onto the ionogel/CuG@PET and CuG@PET with a finger press and then peeled off. The Rsh in each test was measured on a four-point probe resistivity measurement system (Probes tech Co. Ltd., China) at five different locations.

Associated Content The Supporting Information is available free of charge on the ACS Publication website at DOI: Figures and additional instructions for structure of the ionogel/CuG@PET, conductivity of the Cu layers with different Cu depositing time, binding constant (Ka) for Cu and [VEIM][NTf2],


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tape tests, demonstration of a touch-screen panel using ionogel/CuG@PET and large-scale ionogel/CuG@PET fabricated by using soft lithography.

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]; [email protected] Author Contributions Li Chang and Xiqi Zhang contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements This work was supported by the financial support of the National Natural Science Foundation of China (51541304, 51273086, 21404109, 21421061, 91127025, 21431009, 51603211, 51673107), National Research Fund for Fundamental Key Projects (2013CB933000, 2012CB933800), the Key Research Program of the Chinese Academy of Sciences (KJZD-EWM01, KJZD-EW-M03), the 111 project (B14009), Special Doctorial Program Fund from the Ministry of Education of China (20130211110017) and Youth Innovation Promotion Association, CAS (2016026). References


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Copper Grid Composites for High-Performance, Ultra-Stable

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