Omnidirectionally Stretchable and Transparent Graphene Electrodes

Transcription

1 Supporting Information for: Omnidirectionally Stretchable and Transparent Graphene Electrodes Jin Yong Hong,, Wook Kim, Dukhyun Choi, Jing Kong,*, and Ho Seok Park*, School of Chemical Engineering, Sungkyunkwan University, Suwon , Korea Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139, USA Department of Mechanical Engineering, Kyung Hee University, 1 Seocheon-dong, Yongin-si, Gyeonggi-do, , Korea Tel.: ; Fax: ; phs0727@skku.edu Tel.: ; Fax: ; jingkong@mit.edu S1

2 1. Influence of high temperature CVD process on pre-pattered Cu foil morphology. Figure S1. Photographs and OM images of the Cu foil with concentric circle patterns (a) before and (b) after CVD process. Two-dimensional line scan profile taken vertically through the (a) blue and (b) red line on the OM image. It has well known that the annealing of Cu substrate (prior to CVD process) has a strong influence on the surface characteristics (e.g. roughness, grain boundaries, and defects). Wang et al. reported that Cu surface became smmother after prolonged annealing in presence of hydrogen (H2) [Wang, H.; Wang, G.; Bao, P.; Yang, S.; Zhu, W.; Xie, X.; Zhang, W. J. Controllable Synthesis of Submillimeter Single-Crystal Monolayer Graphene Domains on Copper Foils by Suppressing Nucleation, J. Am. Chem. Soc. 2012, 134, ]. Similarly, Gan and Luo also explained that the H2 could reduce Cu surface roughness during annealing step [Gan, L.; Luo, Z. Turning Off Hydrogen to Realize Seeded Growth of S2

3 Subcentimeter Single-Crystal Graphene Grains on Copper, ACS Nano 2013, 7, ]. Recently, Ibrahim and coworkers reported that argon (Ar) and H2 plays an important role in controlling the characteristics of Cu surface morphology [Ibrahim, A.; Akhtar, S.; Atieh, M.; Karnik, R.; Laoui, T. Effects of Annealing on Copper Substrate Surface Morphology and Graphene Growth by Chemical Vapor Deposition, Carbon 2015, 94, ]. On the basis of above studies, it is thus important to perform a comparative experimental study to understand and underpin the influence of high temperature CVD process on Cu surface morphology evolution during annealing stage and investigate its smoothening or roughening effects on the unique concentric circles shaped pre-patterned Cu substrate. In this regard, the following experimental approach was adopted: First, Cu foil (25 μm thick, 99.8%, Alfa Aesar) was pre-patterned by pressing it with a Fresnel lens, which acted as a mold. Second, we annealed the pre-patterned Cu foils at 1000 C with in the presence of Ar (1000 sccm) and H2 (300 sccm), followed by introduction of 30 sccm of methane (CH4) for graphene growth. Lastly, we compared height profile of pre-patterned Cu foil before and after CVD process. This approach enabled us to investigate the impact of annealing process on unique wavy patterns of the Cu foil. The pre-patterned Cu foils before and after CVD process were characterized by photograph, OM and an α-step profilometer, and the results are shown S3

4 in Figure S1. The photograph and OM images clearly show well-defined, concentric circle patterns. In height profiles, there was no significant difference of pattern configuration and parameters. For both samples, the average wavelength and amplitude of the pattern are ca. 350 and 55 μm, respectively. Most of the pattern parameters were remained the same without any smoothening or roughening effects in micrometer scale. Namely, the isotropic ripple (wavy) patterns of the pre-patterned Cu foil were preserved throughout the high temperature CVD process. S4

5 2. Pre-patterned Cu foils with various shapes and configurations. Figure S2. Photograph of (a) the various types and sizes and (b) the selectively connected pre-patterned Cu foils. As shown in Figure S2a, the pre-patterned Cu foils can be formed various shape (1 1, 2 2, and 3 3 formations). Patterned unit was connected directly to another units using serial, parallel, or hybrid formations. The other is selective connection. The omni-directionally S5

6 stretchable electrode can be applied to the articulated section as a joint electrode (Figure S2b). The selectively introduced omni-directionally stretchable electrode creates a connection so that another electrodes can connect to it. It is believed that the whole or partial use of the omni-directionally stretchable electrode can solve the large-area limitations and offers a great feasibility of platforms for a large variety of potential applications, ranging from stretchable devices to electronic components in various wearable integrated systems. S6

7 3. Mechanical property of the textured graphene/pdms film. Figure S3. Stress strain curve up to 40% tensile strain for textured graphene/pdms film at temperatures of 25 C.. Tensile testing was also performed to investigate the mechanical properties of textured graphene/pdms film. As can be seen in the Figure S3, the stress and strain initially increase with a linear relationship up to strain values of 40%. Hence, the Young s modulus (E) was calculated for this linear elastic region (under 40% strain) using Hooke s law, and the average E value of the textured graphene/pdms film was 1.3 MPa. S7

8 4. Thickness effects in the simulations. Figure S4. Thickness effects for effective stress on flat, 1D wavy, and textured graphene films under (a) a x axis load and (b) a y axis load. Due to the too much large wavelength (350 m) and amplitude (55 m) compared to the actual thickness (~1 nm) of multilayered graphene, we could not help modeling the thick graphene with the thickness of the minimum 500 nm. However, we are sure that our simulation is enough to understand the shape effects of graphene in this study even though the thickness of graphene is big different. In Figure S4, we compared the effective stresses at the reference positions for graphene films with different thickness from 10, 5, 2.5 and 0.5 m. For the flat graphene, the effective stresses did not have an effect by film thickness. The effective stresses for 1D wavy graphene were lowest and linearly decreased under a x axis loading condition. However, they were not almost changed by the thickness under a y axis loading condition. In this study, the effective stress of 1D wavy graphene under a y axis load is more S8

9 dominant because we consider omni-directional loading. Thus, it can be understood that the thickness of a 1D wavy film is not important to understand the shape effect of graphene. For the textured graphene film, the effective stresses were the same regardless of uniaxial loading directions and nonlinearly decreased according to the film thickness. Thus, we can expect that the effective stress of the textured graphene might be much lower than that of the film with the thickness of 500 nm. S9

10 5. Reference position selection in the simulations. Figure S5. Stress distributions for (a) 1D wavy and (b) textured graphene films with 4 times larger area under a uniaxial load in the y axis direction. Due to the boundary and edge shape effects, abnormal high stresses could be calculated in the simulation. In Figure 5, the textured graphene showed high effective stresses at the local edges. However, it must be edge shape effects in the model. We thus modeled 4 times larger 1D wavy and textured graphene films and applied a uniaxial load in the y axis direction to understand these edge shape effects. Due to the increased size, the graphene thickness was modeled as 5 m. As shown in Figure S5, 1D wavy graphene showed almost same effective stresses (about 41 1 GPa) on the total film. However, the textured graphene showed high effective stresses (about 17 GPa) at the local edge area and low effective stresses (~ 10 MPa at ~90 % area on the film). Thus, it could be understood that such high effective stresses stem from the edge shape effect in the textured graphene. Generally, we compare the maximum stresses on the film in simulation results. However, when the edge effects are S10

11 serious, we need to eliminate such effects and select the effective stress at reasonable reference positions to compare the effective stresses. Thus, we suitably selected the reference positions in simulation models (black arrows in Figure 6b, c and d). S11

12 6. Mechanical durability of the textured graphene/pdms film. Figure S6. The changes in resistance of textured graphene/pdms film under sequentially repeated stretching tests for 1000 cycles. Uniaxial tensile tests were carried out at a tensile strain of 30%. The mechanical durability of the textured graphene/pdms film was evaluated by repeating stretching cycles (over 1000 times) and they were recorded in Figure S6. Even though there was some scattering of the resistance value during multi-cycle stretching test, the sheet resistances could be reversibly varied from ca kω (free of stretching) to ca kω (30% of tensile strain). Impressively, the textured graphene/pdms film can be effectively transformed with stable and reversible change in the electrical performance, indicating a much better mechanical stability of the graphene sheets. This stable and reversible change in the resistance can be attributed to the lateral expansion of the wavy pattern configuration. S12