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Flexible Electronics
Bio-Inspired Chemical Fabrication of Stretchable Transparent Electrodes You Yu, Yaokang Zhang, Kan Li, Casey Yan, and Zijian Zheng* Stretchable transparent electrodes (STEs) are indispensable components in future deformable optoelectronic devices, which play essential roles in biomedical implants, pointof-care diagnostics and therapy, electronic skins, wearable computing, displays, and photovoltaics. Due to the lack of effective methods to synthesize highly conductive materials that are intrinsically stretchable and transparent, STEs are fabricated by manipulating nonstretchable conducting materials such as carbon nanomaterials,[1–5] conducting polymers,[6–12] metal nanowires (MNWs),[13–18] and metals[19–24] into stretchable and transparent thin films. This can be achieved either by bucking transparent conductors or, more preferably, by assembling conductive materials into interconnected networks. Among these materials, MNW webs and metal nanomeshes are the most competitive because of their high conductivity at large strains. However, high-temperature thermal fusion is required to reduce the contact resistance at the wire junctions of MNW webs, while vacuum-based metal evaporation is needed for the metal nanomeshes. These physical approaches are less favorable for plastic and elastic substrates, especially those nonplanar ones, used in deformable devices. To solve this issue, scientists have aimed at developing low-temperature chemical methods to deposit metals on two-dimensional transparent templates.[25–30] Although those attempts have shown promising results in making highperformance flexible transparent electrodes, their applicability to STEs are yet to demonstrate. In nature, plant leaves possess two-dimensionally ramified and stretchable veins. In photosynthesis, light passes through the vein-free areas and is absorbed by the mesophyll cells of the leaves, while the required substances and produced nutrition are efficiently transported by the veins.[31,32] With billions of years of evolution, the vein architecture is gradually optimized to reach a balance among transparency,
Dr. Y. Yu, Y. Zhang, K. Li, C. Yan, Prof. Z. Zheng Nanotechnology Center Institute of Textiles and Clothing The Hong Kong Polytechnic University Hong Kong, China E-mail: [email protected] Dr. Y. Yu, Y. Zhang, K. Li, C. Yan, Prof. Z. Zheng The Hong Kong Polytechnic University Shenzhen Research Institute Shenzhen 518000, China DOI: 10.1002/smll.201500529 small 2015, DOI: 10.1002/smll.201500529
transportation of substances, and mechanical stability.[33–35] In view of these properties, we believe that veins in leaves are ideal templates for the preparation of STEs, provided that one can render the veins highly conductive. Following this inspiration, we show herein that STEs can be readily fabricated by chemical deposition of metals, such as low-cost Cu, on the surface of veins at ambient conditions. These vein-based transparent electrodes (VTEs) exhibit ultra-low resistance of 0.9 Ω sq−1 at a transmittance of 83%, with a remarkably high figure of merit being 3700. Importantly, the sheet resistance and the optical transmittance of VTEs remain unchanged after 1000 cycles of bending (radius: 1 mm) and stretching (strain: 50%) tests. We demonstrate that these high-performance VTEs are suitable for making (patterned) deformable optoelectronic devices. It should be noted that very recently, Han et al. reported the fabrication of flexible transparent electrodes by thermal evaporation of Ag on natural structures such as veins and spider webs.[29] But they found that their electrodes were not stretchable due to the significant increase in resistance at large strains. In this paper, we report for the first time that our chemical fabrication approach can effectively modify natural veins into highperformance STEs. VTEs were prepared by polymer-assisted metal deposition (PAMD) (Figure 1a).[36] PAMD is a solution-processed chemical method to prepare conformal coating of metal thin films on arbitrary surfaces.[36–39] In a typical experiment, commercially available veins were cleaned briefly with deionized (DI) water and ethanol. They were then immersed into an diluted ethanol solution of poly[2-(methacryloyloxy) ethyl trimethylammonium chloride-co-3-(trimethoxysilyl) propyl methacrylate] [P(METAC-co-MPTS)] and dried. After hydrolysis and curing at 80 °C for 10 min, P(METACco-MPTS) was grafted onto the vein surfaces through the condensation reactions between MPTS and the abundant hydroxyl groups of the veins. The polymer-modified veins were immersed into an aqueous solution of (NH4)2PdCl4 for 15 min, where PdCl42− moieties were immobilized onto the ammonium groups of the polymer layer by ion exchange. Finally, the sample was immersed into a plating bath to for the electroless deposition of Cu. The bare veins and as-prepared VTEs were both freestanding (Figure 1b,d). The veins were mainly composed of two-dimensionally continuous polygonal unit cells that are ≈150 µm thick and 0.5–1.5 mm across. Within each unit cells, there were interconnected sub-networks of 50–70 µm thick with ≈20 µm thick fine brunches (Figure 1c). These
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Figure 1. a) Schematic illustration of the chemical fabrication of bio-inspired VTEs in three steps: (i) polymer coating, (ii) catalyst loading, and (iii) electroless deposition of metal. Digital and SEM images of b,c) the natural veins and d,e) VTEs, respectively. The white dotted lines in c) indicate one unit cell in the veins. Inset in e) shows the cross-section of VTEs.
hierarchical structures were perfectly retained after the metal deposition. The textures on the veins were clearly observed under scanning electron microscope (SEM), indicating that Cu was uniformly and conformably coated on the surface (Figure 1e). The thickness of Cu was estimated to be ≈250 nm by cross-sectional SEM analysis (Figure 1e, inset). These VTEs were highly conductive and transparent. The bulk resistances measured randomly at several spots of the VTEs, as shown in Figure 2a, were ≈4 Ω, which is much lower than that of similar distance of commercially available Indium–Tin Oxide transparent electrodes on glass (≈50 Ω). The sheet resistance (Rs) and transmittance (T) of VTEs mainly depend on the density of the veins. Two typical VTEs with different vein densities were studied. As shown in Figure 2b, VTE I with lower vein density exhibited a Rs of 0.9 Ω sq−1 with a transmittance of 83%. When the vein density increased by ≈10%, the Rs and the transmittance of asmade VTE II decreased to 0.25 Ω sq−1 and 73%, respectively. Such a high transmittance and ultra-low resistance are highly desirable in transparent electrode applications. This will be
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discussed in detail in later sections. Importantly, these VTEs performed stability at in flexible applications. We studied the variations of resistance of VTEs upon bending at different radii of curvatures ranging from 15 to 1 mm (Figure 2c). The normalized resistance (R/R0) remained stable at 1.0 throughout the entire test, where R0 is the resistance of VTEs before bending. Even after 1000 cycles of bending fatigue tests at the smallest radius of curvature, the resistance showed little change, indicating that VTEs are flexible and robust enough to be used as transparent electrodes and interconnects in flexible electronic applications (Figure 2c). Note that the introduction of an interfacial P(METAC-coMPTS) layer in the chemical process not only facilitated the metal deposition, but also significantly improved the adhesion: the Cu coating firmly adhered to the veins and could easily pass the Scotch Tape adhesion test.[38] In contrast, when the Cu layer was deposited by electroless deposition without P(METAC-co-MPTS), or by thermal evaporation, the quality of the Cu thin film was much lower with obvious cracks and uniformed coverage (Figure S1a,b, Supporting Information),
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Figure 2. a) Bulk resistance measurements at different areas of VTEs, showing high conductivity of VTEs. b) Optical transmittance of veins and VTEs. The insets show the optical images of two VTEs with different vein densities. The scale bars are 400 µm. c) Normalized resistances of VTEs at different bending radius (upper) and the fatigue test at the radius of 1 mm (bottom).
and the initial sheet resistance increased to 20 Ω sq−1 at T = 83%. In addition, these low-quality Cu coatings readily delaminated from the surfaces of the veins when the samples were bent or slightly stretched. As a consequence, the resistance increased rapidly by more than three orders of magnitudes after 350 cycles of bending tests at a radius of 3 mm (Figure S1c,d, Supporting Information). small 2015, DOI: 10.1002/smll.201500529
Very importantly, VTEs are outstanding stretchable transparent electrodes. We first evaluated the resistance change of VTEs at different tensile strains of one stretching cycle. To get more precise measurement of the resistance at large strains, a highly conductive liquid eutectic Gallium–Indium alloy was used as soft contacts at the two ends of the VTE sample. VTEs kept stable R/R0 at 1 until 50% strain (Figure 3a). When the sample was further stretched, R/R0 increased rapidly to 1.5 at 60% strain, and the sample became nonconductive at 70% strain. It is worth noting that the transmittance of VTEs, however, was not affected by the strain deformations at all. We further analyzed the tensile fatigue resistance of VTEs. The tests were carried out at 30%, 40%, and 50% maximum strains for 1000 cycles each. Remarkably, the results showed undetectable increase in R/R0 after the fatigue tests (Figure 3b). From microscopic monitoring of the deformation process of the veins, we found that the polygonal unit cells of the veins were deformed: the cells elongated along the strain direction and narrowed perpendicular to the strain direction (Figure 3c–f and Figure S2, Supporting Information). This deformation was fully reversible. As such, the veins remained their integrity during stretching cycles and maintained stable transparency. On the other hand, the Cu coating on the veins remained unaffected in the deformation process until the strains reached the critical value of 50%. Small cracks started to form at the joints of the struts of the veins at strain >50%, and propagated rapidly into large cracks when the tensile strain was further increased to 70%, at which electrical conductivity was fully lost. This phenomenon can be ascribed to the stress concentration occurred at the joints of struts at large strains.[38] The electro-optical performances of the transparent electrodes are evaluated by the figure of merit (F), which can be obtained by fitting the sheet resistance and transmittance data.[24] F of our VTEs is as high as 3700 (dashed line in Figure 4a). Remarkably, this value is significantly higher than all the report values of TCEs in the literature, which are typically in the range of 100–1000 (Figure 4a).[13,16,17,20,40–42] To the best of our knowledge, it is even among the best in all the reported transparent electrodes (including rigid and deformable ones, Figure S3, Supporting Information). It should be noted that VTEs also outperform other STEs in the way that they simultaneously possess constant conductivity and transmittance under large strains. For buckled STEs, one major drawback is the transmittance of the electrode is a function of the applied strain. For reported STEs made of MNW webs or metal nanomeshes, their conductivities typically drop when the strain is larger than 5%. Due to their stable performance at large deformations, VTEs can be readily used for flexible and stretchable optoelectronic devices. For example, we fabricated flexible and stretchable light-emitting diode (LED) circuits using VTEs as transparent interconnects (Figure S4 and Movies S1 and S2, Supporting Information). The circuits could be readily bent or stretched up to 50% strain without detectable change in current density. Alternatively, we could pattern the VTEs with screen printing technique and make transparent twodimensional LED arrays. Briefly speaking, a protecting resist, e.g., transparent and hydrophobic polydimethylsiloxane
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Figure 3. a) Normalized resistance and transmittance of VTEs at different tensile strains. The blue cross shows that the conductivity cannot be detected. b) The tensile fatigue tests of VTEs at 30%, 40%, and 50% maximum strains. c–f) The deformation of veins at different strains ranging from 0%–70%. The white dotted lines indicate the profile of one polygonal unit cell. The scale bars are 1 mm.
(PDMS), was patterned by screen printing onto polymermodified veins and cured. This allowed the selective loading of PdCl4− and subsequent metal deposition. LEDs were glued onto patterned VTEs. As-made LED arrays also worked well at various deformed states (Figure 4b,c). In conclusion, we have reported the development of VTE, a bio-inspired STE that can be fabricated chemically by polymer-assisted metal deposition on veins of natural leaves at ambient conditions. The thin metal layer is uniformly and conformably coated on the surface of the veins to form a hierarchical, junction-free network. As a consequence, asmade VTEs show remarkable conductivity and transmittance (0.9 Ω sq−1 at 83% transmittance) with undetectable performance degradation during severe bending and stretching tests, which significantly outperform other reported STEs in the literature. VTEs have been demonstrated as high-performance flexible and stretchable interconnects and patterned electrodes that are suitable for optoelectronics. It should be noted that current transmittance of VTEs is only 83%, and
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may not be high enough for some applications. Future direction is to develop VTEs exhibiting higher transmittance using other biological templates (perhaps leaves of other kinds). We also believe that these VTEs can be used in a wide variety of deformable devices in the future through the optimization design of man-made vein structures and lithographic scale-up fabrication.
Experimental Section Materials: Veins of Michelia alba DC with length of 10–18 cm and width of 6–12 cm were purchased from the market. Ammonium tetrachloropalladate(II) ((NH4)2PdCl4) and all other chemicals were purchased from Sigma-Aldrich. The functional polymer, (i.e., poly[2-(methacryloyloxy)ethyl trimethylammonium chloride-co3-(trimethoxysilyl)propyl methacrylate], P(METAC-co-MPTS)) was synthesized according to the procedures we reported before,[38] and the ratio of METAC to MPTS was about 3.5:1.
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Figure 4. a) A comparison of the electro-optical performance of VTEs against different best-performing STEs reported in the literatures. These STEs are made from gold nanotrough,[41] gold mesh,[42] graphene,[3] multi-wall carbon nanotube (MWCNT),[43] single-wall carbon nanotube (SWCNT),[2] PEDOT/PSS,[8] silver NWs,[16] and copper NWs.[17] The dashed line shows the fitted result of Rs vs T of VTEs. b,c) Digital images of transparent and flexible LED arrays made with patterned VTEs.
Preparation of VTEs: Veins were cleaned with water and ethanol for several times, and dried at 80 °C for 1 h. Then they were dipped into the ethanol solution of P(METAC-co-MTPS) (0.1 wt%), and dried at 80 °C for 10 min. After that, samples were immediately incubated in an ammonia (95% humidity) atmosphere for ≈3 h at room temperature, and then baked at 80 °C for 15 min. The samples were successively immersed into aqueous solution of (NH4)2PdCl4 (0.5 × 10−3 M) and deionized water for 15 min, respectively. Finally, the PdCl42−-loaded veins were immersed into the electroless deposition (ELD) baths of copper for 15 min, and the VTEs were obtained. The specific procedures for metallization by ELD can be found elsewhere.[37,39] For the patterned VTEs, before catalyst loading, the polymer modified veins were first covered by a shadow mask, and then mixture of PDMS precursor (A:B = 10:1, w/w, Sylgard 184) was screen-printed onto the veins, and cured at 80 °C for 1 h. After that, the PDMS-patterned veins were sequentially immersed into the catalyst solution and plating bath small 2015, DOI: 10.1002/smll.201500529
for catalyst loading and metal deposition, respectively. As a result, the patterned VTEs were obtained. Characterization: The morphology and thickness of veins were studied by scanning electron microscope (SEM, TM3000, Hitachi) and optical microscopy (Eclipse 80i, Nikon). The transmittance spectra were measured by a UV–vis spectrometer (PerkinElmer Lambda 18). Bending and stretching tests were conducted using a x,y-axis stepper motor linear stage (TSA50-C, Zolix) that can be controlled by a computer. For resistance measurement, copper wires (or probes) were connected to veins with silver pastes (or Eutectic Gallium–Indium alloy) and were used as leads for all measurements. I–V characterization was measured by Keithley 2010 Multimeter GPIB remote control. For the sheet resistance measurement, two contacts were drawn by silver paste, separated by a square area of transparent electrode, and then the resistance was measured by Keithley 2010 Multimeter.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements We acknowledge the Research Grant Council of Hong Kong (Project PolyU 5036/13P and PolyU 5030/12P) and The Hong Kong Polytechnic University (Projects G-UB56) for financial support of this work.
[1] J. H. Du, S. F. Pei, L. P. Ma, H. M. Cheng, Adv. Mater. 2014, 26, 1958. [2] L. Cai, J. Li, P. Luan, H. Dong, D. Zhao, Q. Zhang, X. Zhang, M. Tu, Q. Zeng, W. Zhou, S. Xie, Adv. Funct. Mater. 2012, 22, 5238. [3] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, B. H. Hong, Nature 2009, 457, 706. [4] S. H. Chae, W. J. Yu, J. J. Bae, D. L. Duong, D. Perello, H. Y. Jeong, Q. H. Ta, T. H. Ly, Q. A. Vu, M. Yun, X. Duan, Y. H. Lee, Nat. Mater. 2013, 12, 403. [5] D. J. Lipomi, M. Vosgueritchian, B. C. K. Tee, S. L. Hellstrom, J. A. Lee, C. H. Fox, Z. Bao, Nat. Nano 2011, 6, 788. [6] M. S. White, M. Kaltenbrunner, E. D. Glowacki, K. Gutnichenko, G. Kettlgruber, I. Graz, S. Aazou, C. Ulbricht, D. A. M. Egbe, M. C. Miron, Z. Major, M. C. Scharber, T. Sekitani, T. Someya, S. Bauer, N. S. Sariciftci, Nat. Photon. 2013, 7, 811. [7] M. Kaltenbrunner, M. S. White, E. D. Glowacki, T. Sekitani, T. Someya, N. S. Sariciftci, S. Bauer, Nat. Commun. 2012, 3, 1772. [8] D. J. Lipomi, B. C. K. Tee, M. Vosgueritchian, Z. N. Bao, Adv. Mater. 2011, 23, 1771. [9] M. Vosgueritchian, D. J. Lipomi, Z. Bao, Adv. Funct. Mater. 2012, 22, 421. [10] G.-P. Hao, F. Hippauf, M. Oschatz, F. M. Wisser, A. Leifert, W. Nickel, N. Mohamed-Noriega, Z. Zheng, S. Kaskel, ACS Nano 2014, 8, 7138. [11] X. L. Chen, H. J. Lin, P. N. Chen, G. Z. Guan, J. Deng, H. S. Peng, Adv. Mater. 2014, 26, 4444. [12] Z. Zhang, Z. Yang, J. Deng, Y. Zhang, G. Guan, H. Peng, Small 2014, 11, 675. [13] J. J. Liang, L. Li, X. F. Niu, Z. B. Yu, Q. B. Pei, Nat. Photon. 2013, 7, 817. [14] Z. B. Yu, Q. W. Zhang, L. Li, Q. Chen, X. F. Niu, J. Liu, Q. B. Pei, Adv. Mater. 2011, 23, 664. [15] J. Liu, J. Wang, Z. Wang, W. Huang, S. Yu, Angew. Chem. Int. Ed. 2014, 53, 13477.
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[16] J. Wang, C. Yan, W. Kang, P. S. Lee, Nanoscale 2014, 6, 10734. [17] J. Song, J. Li, J. Xu, H. Zeng, Nano Lett. 2014, 14, 6298. [18] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.-Y. Lee, S. Nam, J.-U. Park, Nano Lett. 2013, 13, 2814. [19] S. J. Benight, C. Wang, J. B. H. Tok, Z. Bao, Prog. Polym. Sci. 2013, 38, 1961. [20] J. Liang, L. Li, K. Tong, Z. Ren, W. Hu, X. Niu, Y. Chen, Q. Pei, ACS Nano 2014, 8, 1590. [21] J. A. Rogers, T. Someya, Y. G. Huang, Science 2010, 327, 1603. [22] T. Someya, Stretchable Electronics, Wiley-VCH, Weinheim, Germany, 2012. [23] C. Keplinger, J.-Y. Sun, C. C. Foo, P. Rothemund, G. M. Whitesides, Z. Suo, Science 2013, 341, 984. [24] D. S. Hecht, L. B. Hu, G. Irvin, Adv. Mater. 2011, 23, 1482. [25] T. D. He, A. Z. Xie, D. H. Reneker, Y. Zhu, ACS Nano 2014, 8, 4782. [26] P. C. Hsu, D. S. Kong, S. Wang, H. T. Wang, A. J. Welch, H. Wu, Y. Cui, J. Am. Chem. Soc. 2014, 136, 10593. [27] S. Kiruthika, R. Gupta, K. D. M. Rao, S. Chakraborty, N. Padmavathy, G. U. Kulkarni, J. Mater. Chem. C 2014, 2, 2089. [28] P. C. Hsu, S. Wang, H. Wu, V. K. Narasimhan, D. S. Kong, H. R. Lee, Y. Cui, Nat. Commun. 2013, 4, 3522. [29] B. Han, Y. Huang, R. Li, Q. Peng, J. Luo, K. Pei, A. Herczynski, K. Kempa, Z. Ren, J. Gao, Nat. Commun. 2014, 5, 5674. [30] J. Jensen, F. C. Krebs, Adv. Mater. 2014, 26, 7231. [31] Ü. Niinemets, L. Sack, Progress in Botany (Eds: K. Esser, U. Lüttge, W. Beyschlag, J. Murata, Springer, Berlin, Heidelberg, 2006, Vol. 67, p. 385. [32] P. S. Nobel, L. J. Zaragoza, W. K. Smith, Plant Physiol. 1975, 55, 1067. [33] A. Roth-Nebelsick, D. Uhl, V. Mosbrugger, H. Kerp, Ann. Bot. 2001, 87, 553. [34] T. J. Brodribb, T. S. Feild, L. Sack, Funct. Plant Biol. 2010, 37, 488. [35] K. Esau, Proc. Am. Phiosl. Soc. 1967, 111, 219. [36] Y. Yu, C. Yan, Z. J. Zheng, Adv. Mater. 2014, 26, 5508. [37] X. L. Wang, H. Hu, Y. D. Shen, X. C. Zhou, Z. J. Zheng, Adv. Mater. 2011, 23, 3090. [38] Y. Yu, J. F. Zeng, C. J. Chen, Z. Xie, R. S. Guo, Z. L. Liu, X. C. Zhou, Y. Yang, Z. J. Zheng, Adv. Mater. 2014, 26, 810. [39] R. S. Guo, Y. Yu, Z. Xie, X. Q. Liu, X. C. Zhou, Y. F. Gao, Z. L. Liu, F. Zhou, Y. Yang, Z. J. Zheng, Adv. Mater. 2013, 25, 3343. [40] C. F. Guo, Z. Ren, Mater. Today 2014, DOI:10.1016/j. mattod.2014.08.018. [41] H. Wu, D. S. Kong, Z. C. Ruan, P. C. Hsu, S. Wang, Z. F. Yu, T. J. Carney, L. B. Hu, S. H. Fan, Y. Cui, Nat. Nanotechnol. 2013, 8, 421. [42] C. F. Guo, T. Y. Sun, Q. H. Liu, Z. G. Suo, Z. F. Ren, Nat. Commun. 2014, 5, 4121. [43] T. Chen, H. Peng, M. Durstock, L. Dai, Sci. Rep. 2014, 4, 3612.
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Received: February 23, 2015 Published online:
small 2015, DOI: 10.1002/smll.201500529
Flexible Electronics
Bio-Inspired Chemical Fabrication of Stretchable Transparent Electrodes You Yu, Yaokang Zhang, Kan Li, Casey Yan, and Zijian Zheng* Stretchable transparent electrodes (STEs) are indispensable components in future deformable optoelectronic devices, which play essential roles in biomedical implants, pointof-care diagnostics and therapy, electronic skins, wearable computing, displays, and photovoltaics. Due to the lack of effective methods to synthesize highly conductive materials that are intrinsically stretchable and transparent, STEs are fabricated by manipulating nonstretchable conducting materials such as carbon nanomaterials,[1–5] conducting polymers,[6–12] metal nanowires (MNWs),[13–18] and metals[19–24] into stretchable and transparent thin films. This can be achieved either by bucking transparent conductors or, more preferably, by assembling conductive materials into interconnected networks. Among these materials, MNW webs and metal nanomeshes are the most competitive because of their high conductivity at large strains. However, high-temperature thermal fusion is required to reduce the contact resistance at the wire junctions of MNW webs, while vacuum-based metal evaporation is needed for the metal nanomeshes. These physical approaches are less favorable for plastic and elastic substrates, especially those nonplanar ones, used in deformable devices. To solve this issue, scientists have aimed at developing low-temperature chemical methods to deposit metals on two-dimensional transparent templates.[25–30] Although those attempts have shown promising results in making highperformance flexible transparent electrodes, their applicability to STEs are yet to demonstrate. In nature, plant leaves possess two-dimensionally ramified and stretchable veins. In photosynthesis, light passes through the vein-free areas and is absorbed by the mesophyll cells of the leaves, while the required substances and produced nutrition are efficiently transported by the veins.[31,32] With billions of years of evolution, the vein architecture is gradually optimized to reach a balance among transparency,
Dr. Y. Yu, Y. Zhang, K. Li, C. Yan, Prof. Z. Zheng Nanotechnology Center Institute of Textiles and Clothing The Hong Kong Polytechnic University Hong Kong, China E-mail: [email protected] Dr. Y. Yu, Y. Zhang, K. Li, C. Yan, Prof. Z. Zheng The Hong Kong Polytechnic University Shenzhen Research Institute Shenzhen 518000, China DOI: 10.1002/smll.201500529 small 2015, DOI: 10.1002/smll.201500529
transportation of substances, and mechanical stability.[33–35] In view of these properties, we believe that veins in leaves are ideal templates for the preparation of STEs, provided that one can render the veins highly conductive. Following this inspiration, we show herein that STEs can be readily fabricated by chemical deposition of metals, such as low-cost Cu, on the surface of veins at ambient conditions. These vein-based transparent electrodes (VTEs) exhibit ultra-low resistance of 0.9 Ω sq−1 at a transmittance of 83%, with a remarkably high figure of merit being 3700. Importantly, the sheet resistance and the optical transmittance of VTEs remain unchanged after 1000 cycles of bending (radius: 1 mm) and stretching (strain: 50%) tests. We demonstrate that these high-performance VTEs are suitable for making (patterned) deformable optoelectronic devices. It should be noted that very recently, Han et al. reported the fabrication of flexible transparent electrodes by thermal evaporation of Ag on natural structures such as veins and spider webs.[29] But they found that their electrodes were not stretchable due to the significant increase in resistance at large strains. In this paper, we report for the first time that our chemical fabrication approach can effectively modify natural veins into highperformance STEs. VTEs were prepared by polymer-assisted metal deposition (PAMD) (Figure 1a).[36] PAMD is a solution-processed chemical method to prepare conformal coating of metal thin films on arbitrary surfaces.[36–39] In a typical experiment, commercially available veins were cleaned briefly with deionized (DI) water and ethanol. They were then immersed into an diluted ethanol solution of poly[2-(methacryloyloxy) ethyl trimethylammonium chloride-co-3-(trimethoxysilyl) propyl methacrylate] [P(METAC-co-MPTS)] and dried. After hydrolysis and curing at 80 °C for 10 min, P(METACco-MPTS) was grafted onto the vein surfaces through the condensation reactions between MPTS and the abundant hydroxyl groups of the veins. The polymer-modified veins were immersed into an aqueous solution of (NH4)2PdCl4 for 15 min, where PdCl42− moieties were immobilized onto the ammonium groups of the polymer layer by ion exchange. Finally, the sample was immersed into a plating bath to for the electroless deposition of Cu. The bare veins and as-prepared VTEs were both freestanding (Figure 1b,d). The veins were mainly composed of two-dimensionally continuous polygonal unit cells that are ≈150 µm thick and 0.5–1.5 mm across. Within each unit cells, there were interconnected sub-networks of 50–70 µm thick with ≈20 µm thick fine brunches (Figure 1c). These
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Figure 1. a) Schematic illustration of the chemical fabrication of bio-inspired VTEs in three steps: (i) polymer coating, (ii) catalyst loading, and (iii) electroless deposition of metal. Digital and SEM images of b,c) the natural veins and d,e) VTEs, respectively. The white dotted lines in c) indicate one unit cell in the veins. Inset in e) shows the cross-section of VTEs.
hierarchical structures were perfectly retained after the metal deposition. The textures on the veins were clearly observed under scanning electron microscope (SEM), indicating that Cu was uniformly and conformably coated on the surface (Figure 1e). The thickness of Cu was estimated to be ≈250 nm by cross-sectional SEM analysis (Figure 1e, inset). These VTEs were highly conductive and transparent. The bulk resistances measured randomly at several spots of the VTEs, as shown in Figure 2a, were ≈4 Ω, which is much lower than that of similar distance of commercially available Indium–Tin Oxide transparent electrodes on glass (≈50 Ω). The sheet resistance (Rs) and transmittance (T) of VTEs mainly depend on the density of the veins. Two typical VTEs with different vein densities were studied. As shown in Figure 2b, VTE I with lower vein density exhibited a Rs of 0.9 Ω sq−1 with a transmittance of 83%. When the vein density increased by ≈10%, the Rs and the transmittance of asmade VTE II decreased to 0.25 Ω sq−1 and 73%, respectively. Such a high transmittance and ultra-low resistance are highly desirable in transparent electrode applications. This will be
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discussed in detail in later sections. Importantly, these VTEs performed stability at in flexible applications. We studied the variations of resistance of VTEs upon bending at different radii of curvatures ranging from 15 to 1 mm (Figure 2c). The normalized resistance (R/R0) remained stable at 1.0 throughout the entire test, where R0 is the resistance of VTEs before bending. Even after 1000 cycles of bending fatigue tests at the smallest radius of curvature, the resistance showed little change, indicating that VTEs are flexible and robust enough to be used as transparent electrodes and interconnects in flexible electronic applications (Figure 2c). Note that the introduction of an interfacial P(METAC-coMPTS) layer in the chemical process not only facilitated the metal deposition, but also significantly improved the adhesion: the Cu coating firmly adhered to the veins and could easily pass the Scotch Tape adhesion test.[38] In contrast, when the Cu layer was deposited by electroless deposition without P(METAC-co-MPTS), or by thermal evaporation, the quality of the Cu thin film was much lower with obvious cracks and uniformed coverage (Figure S1a,b, Supporting Information),
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Figure 2. a) Bulk resistance measurements at different areas of VTEs, showing high conductivity of VTEs. b) Optical transmittance of veins and VTEs. The insets show the optical images of two VTEs with different vein densities. The scale bars are 400 µm. c) Normalized resistances of VTEs at different bending radius (upper) and the fatigue test at the radius of 1 mm (bottom).
and the initial sheet resistance increased to 20 Ω sq−1 at T = 83%. In addition, these low-quality Cu coatings readily delaminated from the surfaces of the veins when the samples were bent or slightly stretched. As a consequence, the resistance increased rapidly by more than three orders of magnitudes after 350 cycles of bending tests at a radius of 3 mm (Figure S1c,d, Supporting Information). small 2015, DOI: 10.1002/smll.201500529
Very importantly, VTEs are outstanding stretchable transparent electrodes. We first evaluated the resistance change of VTEs at different tensile strains of one stretching cycle. To get more precise measurement of the resistance at large strains, a highly conductive liquid eutectic Gallium–Indium alloy was used as soft contacts at the two ends of the VTE sample. VTEs kept stable R/R0 at 1 until 50% strain (Figure 3a). When the sample was further stretched, R/R0 increased rapidly to 1.5 at 60% strain, and the sample became nonconductive at 70% strain. It is worth noting that the transmittance of VTEs, however, was not affected by the strain deformations at all. We further analyzed the tensile fatigue resistance of VTEs. The tests were carried out at 30%, 40%, and 50% maximum strains for 1000 cycles each. Remarkably, the results showed undetectable increase in R/R0 after the fatigue tests (Figure 3b). From microscopic monitoring of the deformation process of the veins, we found that the polygonal unit cells of the veins were deformed: the cells elongated along the strain direction and narrowed perpendicular to the strain direction (Figure 3c–f and Figure S2, Supporting Information). This deformation was fully reversible. As such, the veins remained their integrity during stretching cycles and maintained stable transparency. On the other hand, the Cu coating on the veins remained unaffected in the deformation process until the strains reached the critical value of 50%. Small cracks started to form at the joints of the struts of the veins at strain >50%, and propagated rapidly into large cracks when the tensile strain was further increased to 70%, at which electrical conductivity was fully lost. This phenomenon can be ascribed to the stress concentration occurred at the joints of struts at large strains.[38] The electro-optical performances of the transparent electrodes are evaluated by the figure of merit (F), which can be obtained by fitting the sheet resistance and transmittance data.[24] F of our VTEs is as high as 3700 (dashed line in Figure 4a). Remarkably, this value is significantly higher than all the report values of TCEs in the literature, which are typically in the range of 100–1000 (Figure 4a).[13,16,17,20,40–42] To the best of our knowledge, it is even among the best in all the reported transparent electrodes (including rigid and deformable ones, Figure S3, Supporting Information). It should be noted that VTEs also outperform other STEs in the way that they simultaneously possess constant conductivity and transmittance under large strains. For buckled STEs, one major drawback is the transmittance of the electrode is a function of the applied strain. For reported STEs made of MNW webs or metal nanomeshes, their conductivities typically drop when the strain is larger than 5%. Due to their stable performance at large deformations, VTEs can be readily used for flexible and stretchable optoelectronic devices. For example, we fabricated flexible and stretchable light-emitting diode (LED) circuits using VTEs as transparent interconnects (Figure S4 and Movies S1 and S2, Supporting Information). The circuits could be readily bent or stretched up to 50% strain without detectable change in current density. Alternatively, we could pattern the VTEs with screen printing technique and make transparent twodimensional LED arrays. Briefly speaking, a protecting resist, e.g., transparent and hydrophobic polydimethylsiloxane
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Figure 3. a) Normalized resistance and transmittance of VTEs at different tensile strains. The blue cross shows that the conductivity cannot be detected. b) The tensile fatigue tests of VTEs at 30%, 40%, and 50% maximum strains. c–f) The deformation of veins at different strains ranging from 0%–70%. The white dotted lines indicate the profile of one polygonal unit cell. The scale bars are 1 mm.
(PDMS), was patterned by screen printing onto polymermodified veins and cured. This allowed the selective loading of PdCl4− and subsequent metal deposition. LEDs were glued onto patterned VTEs. As-made LED arrays also worked well at various deformed states (Figure 4b,c). In conclusion, we have reported the development of VTE, a bio-inspired STE that can be fabricated chemically by polymer-assisted metal deposition on veins of natural leaves at ambient conditions. The thin metal layer is uniformly and conformably coated on the surface of the veins to form a hierarchical, junction-free network. As a consequence, asmade VTEs show remarkable conductivity and transmittance (0.9 Ω sq−1 at 83% transmittance) with undetectable performance degradation during severe bending and stretching tests, which significantly outperform other reported STEs in the literature. VTEs have been demonstrated as high-performance flexible and stretchable interconnects and patterned electrodes that are suitable for optoelectronics. It should be noted that current transmittance of VTEs is only 83%, and
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may not be high enough for some applications. Future direction is to develop VTEs exhibiting higher transmittance using other biological templates (perhaps leaves of other kinds). We also believe that these VTEs can be used in a wide variety of deformable devices in the future through the optimization design of man-made vein structures and lithographic scale-up fabrication.
Experimental Section Materials: Veins of Michelia alba DC with length of 10–18 cm and width of 6–12 cm were purchased from the market. Ammonium tetrachloropalladate(II) ((NH4)2PdCl4) and all other chemicals were purchased from Sigma-Aldrich. The functional polymer, (i.e., poly[2-(methacryloyloxy)ethyl trimethylammonium chloride-co3-(trimethoxysilyl)propyl methacrylate], P(METAC-co-MPTS)) was synthesized according to the procedures we reported before,[38] and the ratio of METAC to MPTS was about 3.5:1.
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Figure 4. a) A comparison of the electro-optical performance of VTEs against different best-performing STEs reported in the literatures. These STEs are made from gold nanotrough,[41] gold mesh,[42] graphene,[3] multi-wall carbon nanotube (MWCNT),[43] single-wall carbon nanotube (SWCNT),[2] PEDOT/PSS,[8] silver NWs,[16] and copper NWs.[17] The dashed line shows the fitted result of Rs vs T of VTEs. b,c) Digital images of transparent and flexible LED arrays made with patterned VTEs.
Preparation of VTEs: Veins were cleaned with water and ethanol for several times, and dried at 80 °C for 1 h. Then they were dipped into the ethanol solution of P(METAC-co-MTPS) (0.1 wt%), and dried at 80 °C for 10 min. After that, samples were immediately incubated in an ammonia (95% humidity) atmosphere for ≈3 h at room temperature, and then baked at 80 °C for 15 min. The samples were successively immersed into aqueous solution of (NH4)2PdCl4 (0.5 × 10−3 M) and deionized water for 15 min, respectively. Finally, the PdCl42−-loaded veins were immersed into the electroless deposition (ELD) baths of copper for 15 min, and the VTEs were obtained. The specific procedures for metallization by ELD can be found elsewhere.[37,39] For the patterned VTEs, before catalyst loading, the polymer modified veins were first covered by a shadow mask, and then mixture of PDMS precursor (A:B = 10:1, w/w, Sylgard 184) was screen-printed onto the veins, and cured at 80 °C for 1 h. After that, the PDMS-patterned veins were sequentially immersed into the catalyst solution and plating bath small 2015, DOI: 10.1002/smll.201500529
for catalyst loading and metal deposition, respectively. As a result, the patterned VTEs were obtained. Characterization: The morphology and thickness of veins were studied by scanning electron microscope (SEM, TM3000, Hitachi) and optical microscopy (Eclipse 80i, Nikon). The transmittance spectra were measured by a UV–vis spectrometer (PerkinElmer Lambda 18). Bending and stretching tests were conducted using a x,y-axis stepper motor linear stage (TSA50-C, Zolix) that can be controlled by a computer. For resistance measurement, copper wires (or probes) were connected to veins with silver pastes (or Eutectic Gallium–Indium alloy) and were used as leads for all measurements. I–V characterization was measured by Keithley 2010 Multimeter GPIB remote control. For the sheet resistance measurement, two contacts were drawn by silver paste, separated by a square area of transparent electrode, and then the resistance was measured by Keithley 2010 Multimeter.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements We acknowledge the Research Grant Council of Hong Kong (Project PolyU 5036/13P and PolyU 5030/12P) and The Hong Kong Polytechnic University (Projects G-UB56) for financial support of this work.
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Received: February 23, 2015 Published online:
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