Article Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
pubs.acs.org/acsaelm
Enhancing the Intrinsic Stretchability of Micropatterned Gold Film by Covalent Linkage of Carbon Nanotubes for Wearable Electronics Xiaoli Zhao, Shuo Yang, Zijing Sun, Nan Cui, Pengfei Zhao, Qingxin Tang,* Yanhong Tong,* and Yichun Liu* Center for Advanced Optoelectronic Functional Materials Research, and Key Lab of UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China
Downloaded via UNIV OF SOUTHERN INDIANA on July 19, 2019 at 00:42:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Stretchable conductors that can comfortably work even under extreme deformation, such as bending, twisting, folding, or stretching, are indispensable electronic components for constructing soft sensors, memories, transistors, and electronic skins. Gold (Au) film serving as function electrodes, interconnects, and other components has become the most widely used material in a variety of soft devices. It is well-known that micropatterning of stretchable conductors is critical for the development and fabrication of high-integration multifunctional devices and sophisticated electronic devices. However, it is still a huge challenge to realize intrinsically stretchable micropatterned Au film. Here, we develop a facile strategy to achieve high-precision intrinsically stretchable micropatterned electrode by the one-step lift-off photolithography process. SWCNT is designed to covalently link with the Au surface to serve as conductive channels between cracks of the Au pattern for enhancing its intrinsic stretchability and hence ensures the excellent conductivity of the whole hybrid electrode under stretching. The obtained Au/SWCNTs hybrid electrode exhibits many superior advantages, including excellent mechanical stretchability of 175%, ultralow sheet resistance of 1.75 Ω □−1, smooth contact surface with the roughness of 0.73 nm, good conductive uniformity, and outstanding cyclic stretching stability. In addition, the electrode can normally work under different deformation, such as multiple folding, stretching, and conforming to the moving joint of the human body. These results indicate that our strategy offers a new platform for the fabrication of the intrinsic stretchable skin-like conductor, showing the promising potential for low-cost, large-area, and high-integration imperceptible wearable electronics. KEYWORDS: stretchable electrodes, photolithography, wearable electronics, carbon nanotubes, gold film
1. INTRODUCTION With the increasing demand for emerging application areas such as artificial electronic skin, Internet of things, smart displays, and the human−machine interface, a stretchable conductor that can comfortably work even under extreme deformation has become an essential element in nextgeneration wearable and implantable electronics. Previous work on stretchable conductors based on conducting polymers,1 gold films,2 graphene,3,4 carbon nanotubes (CNTs),5,6 metal nanowires,7−9 metal meshes,10 metal nanotrough networks,11 and their composites12,13 has presented strong application potential in emerging wearable and conformal electronics. Among these materials, gold (Au) film serving as function electrodes, interconnects, and other components has become the most widely used material in a variety of soft devices, such as transistors, sensors, memories, cells, and actuators.14−16 Au film offers some superior advantages compared to other conductive materials. (i) An ultrahigh electrical conductivity (107 S m−1) of Au film that is 2 orders higher than carbon-based materials17 helps to minimize power consumption, enabling faster, more efficient devices.18 (ii) Outstanding chemical inertness and biocompatibility ensure biosecurity of the prepared wearable and © XXXX American Chemical Society
implantable devices, while silver or copper fails in maintaining a long-term stability in atmosphere or complex body fluidic environments.19 (iii) Compared to nanostructured conductors such as nanowires, nanomeshes, and nanotroughs, Au film possesses a dense and uniform structure resulting in the formation of excellent conductive uniformity and a very smooth surface, which is beneficial to reduce contact resistance between electrode and active materials and further obtain highperformance devices and circuits.20−23 (iv) The good mechanical robustness of stretchable Au film allows electrical conductivity to be fully recovered after multiple stretchings,24 while CNTs always show partial recover after only once stretching.25 (v) Au film can be easily prepared by vacuum evaporation and can combine with conventional photolithography technology to design high-resolution sophisticated electrode micropatterns,24,26 which is beneficial to realize largescale high-integration multifunction commercial electronic products. Received: April 22, 2019 Accepted: June 21, 2019 Published: June 21, 2019 A
DOI: 10.1021/acsaelm.9b00243 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Electronic Materials Because of the unique advantages of Au film and its significant application prospects in flexible and wearable electronic devices, the fabrication of stretchable Au films has attracted tremendous attention of researchers in the past dozen years.27−33 Previous reports have shown stretchable Au films with a width higher than 1000 μm can maintain the good electrical conductivity at 100% tensile strain29,33 because of the formation of interconnected conductive network of Au islands during stretching.31 It is well-known that micropatterning of stretchable conductors is critical for the development and fabrication of high-integration multifunctional devices and sophisticated electronic devices.34 However, intrinsic micropatterned Au films (≤100 μm) generally have poor stretchability and show extremely high resistance at strains of only a few percent35 because of the inherent rigid nature of Au with high elastic moduli.18 To date, the fabrication of intrinsically stretchable micropatterned Au film is still a huge challenge because of the lack of a scalable fabrication technology. Structural engineering of Au films such as buckling, serpentine, and wave structures has enabled stretchability, but this requires sophisticated fabrication techniques and inevitably limits device density and multifunctional applicability.36−39 In contrast, if an intrinsically stretchable micropatterned Au film can be realized, this will avoid these problems and enable more intimate contact with the human body or various-shaped objects for wearable electronics. The increased contact area and superior conformability would greatly enhance the fidelity of signals acquired from the skin or the surface of objects.40 It is therefore highly desirable to offer a new strategy for improving intrinsic stretchability of the micropatterned Au film, enabling the fabrication of next-generation stretchable skin electronic devices. Herein, we develop a facile strategy to achieve a highprecision intrinsically stretchable micropatterned electrode composed of ultrathin Au film and single-wall carbon nanotubes (SWCNTs) by one-step lift-off photolithography processsomething that has not been reported previously. SWCNT is designed to covalently bond onto the Au film surface through mercaptoethylamine (MEA) linker, which helps form a good conductive channel between cracks of the Au film and then enhances the stretchability of the micropatterned Au film. Based on this novel strategy, the obtained micropatterned electrode not only shows excellent conductive uniformity and good cyclic stretching stability but also has superb mechanical stretchability with the maximum stretching strain of 175%. The versatile electrode can comfortably work under different deformations, such as multiple extreme bending, stretching, and conforming to the moving joint of the human body. This simple manufacturing process offers a new platform for the fabrication of intrinsic stretchable skinlike electrode, showing the promising potential for low-cost, large-area, and high-integration wearable and implantable electronics.
from Chengdu Organic Chemicals Co. Ltd. SWCNT is prepared by catalytically cracking methane with a cobalt catalyst and then oxidizing it in air. The SWCNT length ranged from 5 to 30 μm, and the diameter was 95 wt % was obtained B
DOI: 10.1021/acsaelm.9b00243 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Electronic Materials
Figure 1. Schematic illustration of the fabrication process for stretchable electrodes: (a) Au film; (b) Au pattern. (c, d) Resistance change (ΔR/R0) as a function of applied tensile strain for Au film and Au pattern, respectively. ΔR = R − R0, where R and R0 represent the resistance of the electrode with and without tensile strain, respectively. The insets show optical microscopic images at different tensile strains and their magnified results in small tensile strain.
Figure 2. Preparation and characterization of the stretchable micropatterned Au/SWCNTs electrode. (a) Schematic illustration of the fabrication process and the corresponding optical microscopy images or cross-sectional diagram. Scale bar: 50 μm. (b1, b2) Transmission optical and SEM images of Au/SWCNTs electrode. (b3, b4) AFM images of SWCNTs and Au surface. (c) Comparison of the measured resistance for micropatterned Au, SWCNTs, and Au/ SWCNTs electrode under stretching strain up to 100%.
shows an electrical disconnect at a very small tensile strain (around 1%), which is consistent with the previously reports.42 In Figure 1d, the Au pattern has a resistance change of 0 to 7 × 105 at a strain of 0−1%, and the resistance change is 0.95 when the tensile strain is 0.6%. The key reason is that although there are many longitudinal cracks in the 3 mm wide Au film during the stretching process, the crack does not pass through the entire width of the track as shown in the optical insets of Figure 1c. In other words, during deformation, the formation of interconnected conductive network of islands ensures the good conductivity of the Au film, while the crack has pass through the entire width of Au pattern leading to the poor conductivity. This is because the change in resistance is related to the microcrack density and size (Figures S3 and S4). 3.2. Design and Fabrication of the Stretchable Micropatterned Au/SWCNTs Electrode. Inspired by the formative interconnected conductive network of the Au film under stretching, SWCNT is designed to connect to Au surface for the formation of conductive channels between cracks of the Au pattern. Figure 2a illustrates the idea and main fabrication scheme for our stretchable micropatterned Au/SWCNTs electrode. In our experiments, SWCNT is selected as conductive channels because of its good stretchability and conductivity.43,44 Initially, photoresist was patterned on OTS/ Si substrate by conventional photolithography (Figure 2a, I), and then an ultrathin (25 nm) Au film was deposited on it (Figure 2a, II). Next, MEA and SWCNTs were connected successively on the Au surface by a simple dip-coating and spray-coating process (Figure 2a, III to IV). By changing the number of sprays, we can control the number of layers of SWCNTs on Au surface. After removal of the residual photoresist, the Au/SWCNTs pattern can be formed on the OTS/Si substrate via a lift-off process (Figure 2a, V). Subsequently, the elastic PDMS was spin-coated onto the sample to make Au/SWCNTs pattern embedded in PDMS for further enhancing the stretchability (Figure 2a, VI). Finally, the whole embedded Au/SWCNTs pattern was successfully peeled off from OTS/Si substrate and flipped over to form a micropatterned stretchable electrode (Figure 2a, VII to VIII). Its cross-sectional diagram clearly shows the structure of hybrid
electrode. The corresponding real optical microscopy images are shown in the insets of Figure 2a. No additional adhesive or glue was applied in our experiments. No solvent or water was introduced in the peeling processes. Such an all-dry manufacturing process not only avoids the use of toxic or environmentally harmful solvents but also produces a clean and smooth contact interface for further wearable electronic device fabrication. Based on the novel strategy, high-precision Au/SWCNTs array can be easily obtained. Figure 2b1−b4 shows a typical Au/SWCNTs array with the width of 30 μm. The number of layers of SWCNTs on Au surface is 25 layers. The transmission optical image clearly demonstrates a typical electrode array without wrinkles and cracks on its surface (Figure 2b1), which enables the intimate contact with the human body or variousshaped objects. Figure 2b2 presents SEM images of the electrode array. As shown in the magnified SEM image, the SWCNTs network is compactly and uniformly distributed on the surface of the Au array, which ensures the good charge transfer of the electrode. It can be clearly see that the electrode array is well-defined with a sharp edge, and there is no residual SWCNTs in void areas. Figures 2b3 and 2b4 show the morphology of the Au/SWCNTs hybrid electrode of the upper and lower surface, respectively. It is worth mentioning that the upper surface of hybrid electrode has very low roughness (0.73 nm), which is much lower than that of the lower surface (10.5 nm). This not only eliminates the open spaces caused by SWCNT itself45−50 but also can effectively improve the contact quality between electrode and active materials, which is beneficial to further fabricate high-performance wearable electronic products.20−23 Figure 2c presents the comparison results of the measured resistance for micropatterned Au, SWCNTs, and Au/SWCNTs electrode under stretching strain up to 100%. Their sizes are controlled to be the same with the width of 100 μm and the length of 10 mm. Compared to the micropatterned Au and SWCNTs resistors and normalized resistance changes, the Au/SWCNTs hybrid electrode C
DOI: 10.1021/acsaelm.9b00243 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Electronic Materials
Figure 3. Effect of MEA modification on Au/SWCNTs hybrid electrode. Schematic illustrations and the corresponding AFM images before and after the Scotch-tape detachment test: (a) with MEA modification; (b) without MEA modification. (c) Schematic diagram of the covalent linking of Au and SWCNTs. (d) XPS spectra of covalently bonded SWCNTs on the Au surface.
Figure 4. Effect of MEA modification on the stretchability of Au/SWCNTs pattern. (a, b) Schematic illustrations and the corresponding transmission optical images of Au/SWCNTs pattern at different tensile strains with and without MEA modification. (c) Resistance change (ΔR/ R0) for Au/SWCNTs pattern in response to tensile strain. The inset shows the magnified figure at 1% (top) and 100% (bottom) strain.
demonstrates excellent stretchability and completely can withstand 100% tensile strain. The Au/SWCNTs hybrid electrode exhibits excellent stretchability. As tensile strain ranges from 0 to 100%, resistance changes from 175 Ω to 1184 kΩ (Figure 2c), which shows a strong potential for the application of stretchable strain sensor. The resistance of the micropatterned Au/SWCNTs hybrid electrode shows a slow increase with increasing tensile strain, while Au and SWCNTs show a rapid increase at a small tensile strain of 1% and 38%, respectively. The clear contrast captured in Figure 3c proves that SWCNTs in the hybrid electrode plays a crucial role as an alternative current path under highly stretchable conditions. Both the conducting components of Au and SWCNTs allow simultaneous charge transport in this hybrid geometry, each complementing the disadvantages of the other component. 3.3. Effect of MEA Modification on Au/SWCNTs Electrode. In our strategy, the MEA molecule is used as the intermediate layer to form a strong interaction between Au and SWCNTs, which not only avoids SWCNTs shedding in
photoresist removal process but also increases the mechanical robustness of the hybrid electrode. To demonstrate the role of MEA modification, we performed a Scotch-tape detachment test. Figures 3a and 3b show the comparison AFM images of the pristine and MEA-modified Au/SWCNTs electrode before and after tape peeling. The MEA-modified Au/SWCNTs electrode exhibits a high stability against a Scotch-tape detachment test, and SWCNTs are not separated from the Au surface (Figure 3a). However, for the pristine Au/ SWCNTs electrode without MEA modification, almost all of SWCNTs are peeling off from the Au surface (Figure 3b). These results suggest that MEA helps form a strong interaction between the Au electrode and SWCNTs, which is one key to realizing the highly stretchable and durable electrode. Figure 3c shows the schematic diagram of covalently linked Au and SWCNTs with MEA modification. The SH− and NH2− groups of MEA are used as functional sites to covalently link Au and SWCNTs, respectively.51−53 XPS measurements are utilized to identify the existence of MEA (Figure 3d). XPS D
DOI: 10.1021/acsaelm.9b00243 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Electronic Materials
Figure 5. Optical/AFM images and resistance change of stretchable micropatterned Au/SWCNTs electrode: (a1−a3) for different line width with a fixed length of 10 mm; (b1−b3) for different SWCNTs layers with a fixed length of 10 mm.
SWCNTs pattern with and without MEA modification at 150% tensile strain, we found that the Au/SWCNTs patterned electrode without MEA modification showed severe delamination, while the electrode with MEA modification did not detach from PDMS layer (Figure S5). All these features enable our strategy to be very promising for application in skininspired and imperceptible wearable electronics. 3.4. Mechanical Properties of Micropatterned Au/ SWCNTs Electrode. It is worth mentioning that our strategy integrates photolithography techniques, which can produce small-size, versatile, and high-precision electrode micropatterns.24,56 This provides us an effective approach to systematically explore the stretchability of the Au/SWCNTs electrode with different line widths, which is extremely important for future wearable electronic integration. As demonstrated in Figure 5a1, the well-defined Au/SWCNTs electrodes with various line widths of 30, 50, and 100 μm can be obtained based on photolithography. Figure 5a2 illustrates their resistance change as a function of tensile strain up to 100%. With the increasing line width from 30 to 100 μm, the resistance change gradually stabilizes under stretching. It can be clearly observed that the wider electrode shows smaller resistance change (Figure 5a3). The narrow electrode with 30 μm line width shows relatively larger change due to the decreased contact points between nanotubes (Figure 5a3). Nevertheless, this value still far exceeds the elongation at break of single Au pattern (
