Supporting Information

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1 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Supporting Information for Adv. Mater., DOI: /adma Highly Stretchable Patterned Gold Electrodes Made of Au Nanosheets Geon Dae Moon, Guh-Hwan Lim, Jun Hyuk Song, Minkwan Shin, Taekyung Yu, Byungkwon Lim,* and Unyong Jeong*

2 Supporting Information Highly Stretchable Patterned Gold Electrodes Made of Au Nanosheets by Geon Dae Moon, 1, Guh-Hwan Lim, 2, Jun Hyuk Song, 1 Minkwan Shin, 1 Taekyung Yu, 2 Byungkwon Lim, 2, * and Unyong Jeong 1, * They equally contributed to this work. [*] Prof. U. Jeong, Dr. G. D. Moon, J. H. Song, M. Shin, Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seoul, Korea ujeong@yonsei.ac.kr Prof. B. Lim, G.-H. Lim, Dr. T. Yu School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon , Korea blim@skku.edu Experimental Synthesis of Au nanosheets. In a typical synthesis, 1.7 mg of L-arginine (Aldrich) was dissolved in 5 ml of water and the solution was heated to 95 C. Meanwhile, 2 ml of aqueous solution containing 13.5 mg of hydrogen tetrachloroaurate trihydrate (HAuCl 4 3H 2 O, Aldrich) was rapidly injected into the reactor using a pipette. The reactor was kept at 95 C for 2 h, and then

3 cooled to room temperature in air. Preparation of Ecoflex and PDMS substrates. An Ecoflex substrate was prepared by mixing Ecoflex monomer (Smooth-On Inc.) and curing agent (V:V = 1:1) to make 50 g of solution. This solution was poured into a petri dish and cured for 12 h at room temperature. The PDMS substrate was made of poly(dimethylsiloxane) by mixing the base monomer and a curing agent at 10:1 (w:w) (Sylgard 184, Dow Corning). This PDMS solution was poured into a petri dish and was left in air for 2 h to remove air voids. The PDMS substrate was thermally cured at 80 C for 1 day and was then peeled off. Preparation of patterned PDMS stamps. SU-8 (MicroChem) was spin-coated on a plasma-treated silicon wafer at 1000 rpm for 30 s. The wafer was then soft-baked at 90 C for 30 min and exposed through masks under 365-nm of UV light at 1800 mj/cm 2. The wafer was baked at 90 C for 10 min again. The resist was developed by PGMEA (MicroChem.). The elastomer stamp was made of poly(dimethylsiloxane) (PDMS) by mixing the base monomer and a curing agent at 10:1 (w:w) (Sylgard 184, Dow Corning). This PDMS solution was poured into the SU-8 mold and left in air for 2 h to remove air voids. The PDMS stamp was thermally cured at 80 C for 1 day and peeled off of the wafer. Fabrication of the Au nanosheet films and patterns. The monolayer film of the Au nanosheets was obtained following the process previously reported by our group (ACS Nano 2011, 5, 8600). A clean petri dish filled with D.I. water was used as a reservoir to float the Au nanosheets on it. The Au nanosheet dispersed in 1-butanol was dropped by a pipette on the water surface until a robust film was formed which would be seen by the naked eye. Once the monolayer film was formed at the air-water interface, the substrates (Si wafer, paper, PDMS, Ecoflex) were put into contact or scooped up to transfer the Au nanosheet film. This process was repeated to obtain the multi-layer 2

4 films of the Au nanosheet. The patterned Au electrode was fabricated by touching a patterned PDMS stamp to the Au nanosheet film and transferring the pattern to flat substrates (Ecoflex, PDMS). Preparation of P3HT fiber. P3HT (poly(3-hexylthiophene)) fiber was electrospun using a mixed solution of P3HT (M w =87,000, 95% regioregularity):pcl (M w =80,000) (7:3, w:w) in chloroform. The electrospinning of P3HT was carried out with a single nozzle (inner diameter = 110 um) at 18 kv. The P3HT fibers were collected on a rectangular-patterned Au nanosheet electrode on an Ecoflex substrate to test the charge injection during stretching. Fabrication of the stretchable LED. The line-patterned PDMS stamp was used to fabricate stretchable LED device. After transferring the Au nanosheet line pattern to a flat Ecoflex substrate, the Au electrode was connected to LED light bulbs in a row. The two terminals of the LED chips were pinned to the Au nanosheet electrodes and a small amount of liquid metal, EGaIn (eutectic Ga-In), was used to guarantee the contact between the electrode and the terminals. In the current (I) vs. voltage (V) curve, the current was measured at the two LED terminals and the voltage (0 ~ 5V) was directed to the Au nanosheet electrodes. The applied voltage ranged from 0 to 5V. Characterization. The shape of the Au nanosheets and the morphology of the nanosheet films were investigated with a scanning electron microscope (SEM, Jeol JSM-6701F). TEM studies were carried out with a JEOL 2100F microscope operated at an acceleration voltage of 200 kv. The powder XRD pattern was obtained with a Bruker D8 Discover apparatus. The measurement of the electrical properties was carried out using an Agilent 4156A device. The thickness of the Au nanosheet was measured by atomic force microscopy (AFM, Model Dimension 3100, Digital Instruments Co.). 3

5 Figure S1. AFM image showing the height profile of a Au nanosheet. Figure S2. SEM images of Au nanosheet films transferred from the water surface to Si substrates. The number of transfers was 2 (A) and 3 (B). 4

6 Figure S3. Surface profile of the 8-times transferred Au nanosheet film. The profile was obtained after pressing with a plastic roller (a centrifuge tube). The root-mean-square (rms) roughness were 241 nm. 5

7 Figure S4. Changes in the electrical resistivity (A) and the sheet resistance (B) with the number of transfers of the Au nanosheet film from the water surface to the Si substrates. The inset (A) shows the changes of the electrical resistivity as a function of the annealing temperature (for 1 h). The electrode distance for the measurement was 500 µm. 6

8 Figure S5. Au nanosheet film before (A) and after (B) thermal annealing at 250 C for 1h 7

9 Figure S6. (A,B) Resistivity of the Au nanosheet film transferred (four times) to a paper substrate (bank note) during the bending tests. (A) Resistivity of the Au nanosheet film at various bending radii. The inset images show the bending state corresponding to the bending radii. (B) Resistivity of the Au nanosheet film on a paper substrate during folding tests with 1,000 trials (1/R = ~ 18 cm -1 ). The inset shows SEM images of the Au nanosheet film at the folded area. (C) An enlarged SEM image showing the local area of the folded Au nanosheet film. 8

10 Figure S7. Images showing the bond strength between the Au nanosheet film and an ecoflex substrate. The Au nanosheet film keeps its adhesion to the substrate: (A) the nanosheet film, (B) after taping a sticky tape, and (C) after peeling off the sticky tape. 9

11 Figure S8. Opitcal microscopy images of the Au nanosheets in the center of the ecoflex substrate at o% strain (A) and at 300% strain (B). (C) A calibration curve of the real strain in the center versus the apparent strain applied to the substrate. No meaningful difference was observed between the local strain in the central area of the rubber and the strain applied to the substrate. 10

12 Figure S9. (A) Electrical resistivity as a function of the strain applied to the Au nanosheet film on a PDMS substrate. Thermal annealing (indicated in the label) led to better electrical conductance. (B) Electrical resistivity of the Au nanosheet films as a function of the strain according to the number of transfers to the PDMS stamps. The films were annealed at 150 C for 1h before the measurement. The numbers in the box indicate the number of transfers. 11

13 Figure S10. Electrical resistivity of the Au nanosheet film consisting of five layers during repeated stretching-releasing cycles at 30% and 40 % strain levels. Figure S11. Surface morphology change of a Au nanosheet during stretching from 0% to 50% strain. 12

14 Figure S12. Microscope images of a Au nanosheet film on a square-patterned PDMS (top) and transferred to a flat PDMS substrate (bottom). 13

15 Figure S13. Optical microscopy images of rectangular Au patterns thermally evaporated on PDMS substrates. (A) At 5% strain in the longitudinal direction and (B) at 10% strain in the transversal direction. 14

16 Figure S14. Optical images of the line-patterned Au nanosheet electrode under strains along the longitudinal (A, B, C for 0, 50, 70%) and the transversal directions (D, E, F for 0, 100, 150%), respectively. 15

17 Figure S15. LED bulb illumination using the line-patterned Au nanosheet electrode. Optical microscope images of the electrode corresponding to 60% strain (A), 120% strain (B), and in the recovered state (C). The insets are the corresponding images of the LED illumination. (D) I-V characteristics between the electrodes connected to the LED bulb during the stretching cycle. 16

18 Figure S16. Optical images of the rectangular-patterned Au nanosheet electrode under strains along the longitudinal (A, B, C for 0, 40, 60%) and the transversal directions (D, E, F for 0, 60, 80%), respectively. 17

19 Figure S17. Electrical resistivity of the line-patterned Au electrode as a function of the number of cycles at a strain of 120% (transversal) and 50% (longitudinal). The resistivity values were measured upon stretching and relaxing corresponding to every 50 cycles. 18

20 Figure S18. Electrical resistivity of the rectangular-patterned Au electrode as a function of the number of cycles at a strain of 70% (transversal) and 50% (longitudinal). The resistivity values were measured upon stretching and relaxing corresponding to every 50 cycles. Figure S19. I-V characteristic curve of electrospun P3HT fibers onto a gold electrode on a silicon wafer. 19