Stretchable, Transparent, and Stretch-Unresponsive Capacitive Touch

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Stretchable, Transparent and Stretch-Unresponsive Capacitive Touch Sensor Array with Selectively Patterned Silver Nanowires/Reduced Graphene Oxide Electrodes Taeyoung Choi, Byeong-Ung Hwang, Bo-Yeong Kim, Tran Quang Trung, Yun Hyoung Nam, Do-Nyun Kim, Kilho Eom, and Nae-Eung Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Stretchable, Transparent and Stretch-Unresponsive Capacitive Touch Sensor Array with Selectively Patterned Silver Nanowires/Reduced Graphene Oxide Electrodes Tae Young Choi†, ‡, Byeong-Ung Hwang§, Bo-Yeong Kim⊥, Tran Quang Trung§, Yun Hyoung Namǁ, Do-Nyun Kimǁ, Kilho EomΟ, Nae-Eung Lee§,⊥, □,* †

Samsung Institute of Technology (SSIT), Samsung Display Co., Ltd., Yongin, Gyeonggi-Do, 17113, Korea ‡

College of Information & Communication Engineering, Sungkyunkwan University, Suwon, Gyeonggi-Do 16419, Korea §

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, Gyeonggi-Do 16419, Korea ⊥

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, Gyeonggi-Do 16419, Korea

ǁ

Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanak-ro 1, Gwanakgu, Seoul, 08826, Republic of Korea

Ο

College of Sports Science, Sungkyunkwan University, Suwon, Gyeonggi-Do 16419, Korea

□

Samsung Advanced Institute of Health Sciences and Technology (SAIHST), Sungkyunkwan University, Suwon, Gyeonggi-Do 16419, Korea

KEYWORDS. Stretchable capacitive touch sensor, stretchable and transparent electrode, silver nanowire, reduced graphene oxide, stretchable sensor array, selective patterning

ACS Paragon Plus Environment

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ABSTRACT. Stretchable and transparent touch sensors are essential input devices for future stretchable transparent electronics. Capacitive touch sensors with a simple structure of only two electrodes and one dielectric are an established technology in current rigid electronics. However, the development of stretchable and transparent capacitive touch sensors has been limited due to changes in capacitance resulting from dimensional changes in elastomeric dielectrics and difficulty in obtaining stretchable transparent electrodes that are stable under large strains. Herein, a stretch-unresponsive stretchable and transparent capacitive touch sensor array was demonstrated by employing stretchable and transparent electrodes with a simple selective-patterning process and by carefully selecting dielectric and substrate materials with low strain responsivity. A selective-patterning process was used to embed a stretchable and transparent silver nanowires/reduced graphene oxide (AgNWs/rGO) electrode line into a polyurethane (PU) dielectric layer on a polydimethylsiloxane (PDMS) substrate using oxygen plasma treatment. This method provides the ability to directly fabricate thin film electrode lines on elastomeric substrates and can be used in conventional processes employed in stretchable electronics. We used a dielectric (PU) with a Poisson’s ratio smaller than that of the substrate (PDMS), which prevented changes in the capacitance resulting from stretching of the sensor. The stretch-unresponsive touch sensing capability of our transparent and stretchable capacitive touch sensor has great potential in wearable electronics and human-machine interfaces.

1. INTRODUCTION Stretchable electronic devices have received a great deal of attention for next-generation wearable electronics such as sensors1-8, energy harvesters9, energy storage10, electronic skins11,


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actuators12, electronic circuits13-15 and displays16-18. Wearable electronics, like rigid electronics, involve many interactive systems between humans and electronic devices including input devices like touch screens6-8, keyboards19 or microphones20 and output devices like displays16, speakers20, and actuators12. Touch screens are the most popular input device, and rigid, transparent, capacitive touch sensors have been commonly used for many mobile devices such as smart phones and tablets. Although transparent capacitive touch sensors are established technologies in rigid electronics, stretchable transparent capacitive touch sensors that require conformality in wearable electronics have been rarely reported due to the two following reasons: complicated fabrication processes and unstable capacitance under large stains. First, the process used to fabricate stretchable and transparent electrodes with good electrical stability under stretching is complex. Meshes of silver nanowires (AgNWs) and various AgNWs composites have been developed for replacing transparent conducting oxides in stretchable electronics due to their cost effectiveness, low resistance, large mechanical deformability, and high optical transparency. However, most of this work has required a complex transfer process for the AgNWs thin films, which include coating on rigid substrates such as wafers or glasses, embedding those films into elastomeric materials like polydimethylsiloxane (PDMS) or polyurethane (PU), and removing the rigid substrates5-7. Those processes are inefficient because of the additional rigid substrate and transfer process and because the transfer processes cannot be repeated on the same substrate. This means that the multi-layered and complex structures required in the commercial electronics industry are unattainable. Compared to the transfer or embedding methods for stretchable and transparent electrodes, the selectivepatterning process has the advantages of enabling direct formation of a thin film layer on the


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elastomeric substrate and the ability to use conventional processes that exist in current fabrication facilities. Secondly, the capacitance under stretching is not stable because of dimensional changes in the dielectric layer under stretching. Stability in the touch capacitance signal under large strains is one of the most important electrical characteristics for capacitive touch sensors used as input devices in wearable electronics. However, most reported stretchable touch sensors have shown capacitance changes induced by straining as well as capacitive touch sensing under stains, which will complicate the touch sensing signal under stretched conditions6,7,21. In other words, large elongations in stretchable dielectrics under stretching lead to additional capacitance changes during touch sensing22. In this case, the deformation of stretchable dielectrics under stretching is dominated by the Poisson’s ratios of the dielectrics and substrates. Uniaxial, planar strain induces a contraction in the dielectrics in the perpendicular axes, which is controlled by the Poisson’s ratios of the materials. This results in a decrease in the dielectric thickness and, in turn, an increase in the capacitance of the touch sensor. Consequently, interference of the touch signal caused by the capacitance change under stretching is inevitable in stretchable capacitive touch sensors. To overcome these two drawbacks, we adopted a selective-patterning process for direct coating of stretchable transparent electrodes on the elastomeric substrate during device fabrication, and we carefully chose stretchable dielectric and substrate materials by considering their Poisson’s ratios to reduce dielectric deformation and the corresponding capacitance change induced by stretching. A AgNWs/reduced graphene oxide (rGO) hybrid material was used as a stretchable and transparent electrode and was embedded into PU (the dielectric layer) to improve stretchability. A selective-patterning technique was introduced for direct coating of the


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electrodes on an elastomeric substrate to simplify the fabrication process. The one-step selfpatterning process allowed for consecutive direct coatings of AgNWs, rGO and PU solutions on an elastomeric PDMS substrate. The low Poisson’s ratio of the dielectric PU thin films compared to the high Poisson’s ratio of the PDMS substrates minimized changes in the dielectric thickness. As a result, stretch-unresponsive touch signals could be obtained with minimal changes in the capacitance by stretching. The devices showed very small responsivity to stretching strain. Using these approaches, we additionally demonstrated a 5×5 array of stretchable and transparent capacitive touch sensors. These devices can be used to sense finger touches for wearable electronic devices and human-machine interfaces.

2. RESULTS AND DISCUSSION To achieve highly stretchable electrodes for capacitive touch sensors, various thin films of AgNW composites on PDMS substrates were investigated including AgNWs, AgNWs/PU and AgNWs/rGO/PU structures. The PU layer in AgNWs/PU and AgNWs/rGO/PU films was used as the dielectric layer in a capacitor structure. Here, AgNWs and AgNWs/rGO were embedded into the PU film. AgNW thin film electrodes were fabricated by spin-coating a AgNW dispersion on the PDMS substrate followed by thermal annealing. AgNW electrodes embedded into PU thin films were fabricated by spin-coating and annealing of water-based PU dispersions on the AgNW thin film/PDMS substrate. During the spin-coating process, the PU dispersion smeared into the network of AgNWs, resulting in imbedded AgNWs/PU thin films. Details of the fabrication process are provided in the Experimental Section. The field-emission scanning electron microscopy (FE-SEM) image in Figure 1(a) shows the successful imbedding of AgNWs


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into the PU thin film. The FE-SEM image was acquired from the sample with 300-nm-thick PU film coated on the AgNW thin film to confirm the embedding of AgNWs into PU thin film. White and red arrows showing each part of the AgNWs upward to the surface of the PU and downward the bottom of the PU, respectively, indicate that the AgNWs were slanted and well embedded into the PU only by spin-coating. And it was observed that while the AgNWs located near the top surface of PU film showed a bight contrast, the AgNWs located near the bottom surface or deep inside the PU film showed a dim contrast. Similarly, the AgNWs/rGO electrode embedded into the PU layer was fabricated by consecutive spin-coating of rGO and PU dispersions on a AgNW thin film. The rGO dispersion was spin-coated on the substrate right after the original GO was reduced using hydrazine. The data in Figure 1(b) depicts a comparison of the sheet resistances of various thin films of AgNW, AgNWs/PU and AgNWs/rGO/PU thin films under strains up to 50%. The sheet resistance of the AgNWs thin film drastically increased at 30% stretching strain. The AgNWs/PU thin film showed an increase in the sheet resistance by two orders of magnitude at 30% strain compared to that of the unstrained AgNW thin film. The AgNWs/PU thin film finally lost its conductive properties at 40% strain. Although the addition of PU relieved the rate of increase in sheet resistance with respect to the applied strain by forming a percolation network of AgNWs in PU, the AgNW network was lost at 40% stretching. It is because the initial sheet resistance at the unstrained condition increased drastically due to an increase in the contact resistance at the AgNW junctions by permeating of PU solution through adjacent AgNWs. On the other hand, the sheet resistance of the AgNWs/rGO/PU thin film settled in the range of 1~8 Ω/□ up to 50% strain and had a high optical transmittance of 92.7%. Graphene oxide(GO)-soldered AgNW percolation networks have shown better stretchability with high toughness when GO is used as a


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soldering material23. As seen in the top-view FE-SEM image of the AgNWs/rGO thin film in Figure 1(c), the rGO flakes tie several AgNWs together at AgNW junctions after spin-coating the rGO dispersion on the AgNW film followed by annealing. Additionally, the highly conductive rGO can provide current paths among AgNWs, resulting in a low sheet resistance compared to AgNWs/PU thin films. Since the stability of the AgNWs/rGO/PU thin film is good under different stretching strains, we conducted cyclic stretching tests on the AgNWs/rGO/PU thin film. Figure 1(d) shows that the conductance of the AgNWs/rGO/PU thin film was stable under 1,000 cycles of stretching at a strain of 30%. These results indicate that the addition of rGO flakes plays an important role in improving the stability of stretchable and transparent AgNWs/rGO thin film electrodes embedded into PU. A single stretchable capacitive touch sensor with a AgNWs/rGO electrode embedded into PU (as a dielectric sensing layer) was fabricated on PDMS substrates. The cross-sectional view of the capacitive touch sensor with AgNWs/rGO/PU films on a PDMS substrate is depicted in Figure 2(a). A AgNWs/rGO percolation network electrode was embedded in the PU on the top and bottom PDMS substrates to form a AgNWs/rGO/PU structure. Then, these two layers of PU from the top and bottom substrate were combined to make a capacitor device. The schematic in Figure S1 in Supporting Information presents the fabrication process used for the single capacitive touch sensor, which consists of the formation of AgNWs/rGO/PU nanocomposite films on two separate PDMS substrates followed by hot pressing of those substrates. The AgNWs/rGO thin films act as electrodes, and the bonded PU layer serves as a dielectric layer in the AgNWs/rGO/PU. The whole thin film process is simple compared to the transfer method, which requires coating of the solutions on a rigid substrate and an additional transferring process to an elastomeric substrate5,6,21. After the hot-pressing process, electrical contacts were added to


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the device by using a eutectic Ga-In alloy (EGaIn) and a Au textile for measurements. More detailed experimental conditions are provided in the Experimental Section. The touch sensing mechanism of the general (mutual) capacitive touch sensor is illustrated in Figure 2(b). The fringing electric field between the top and bottom electrodes is partially distributed to the touched finger, resulting in a decrease in the capacitance in the touching state (bottom) compared to the non-touching state (top). To evaluate the touch sensing characteristics of the device, touch sensing signals were measured under stretched conditions. The data in Figure 2(c) show the capacitance change in the single stretchable and transparent capacitive touch sensor measured under applied strains up to 60% with and without touching. The strain responsivity of a single capacitive touch sensor was measured using a custom-built stretching tester (see the inset in Figure 2(c)). As shown in Figure 2(c), the capacitance was nearly unchanged with respect to the stretching strain up to 60% with and without touching. Interestingly, the results in Figure 2(c) indicate that at each stretching strain, the capacitance change induced by touching (22 to 24%) was unchanged. This result indicates that this sensor has the ability to sense touch even under stretched conditions. The strain responsivity was quantified in terms of the strain gauge factor, ∆C/Coε. Here, ∆C is the capacitance change, Co is the initial capacitance before straining, and ε is the applied strain to the sensor. Based on the data in Figure 2(c), the strain gauge factor was about 0.10 to 0.13, which is surprisingly low compared to other reported results5,6,21. In addition to the stretching test, the bending tests under compressive strain were also performed and the data were presented in Figure S2 in Supporting Information. The touch sensor also showed a decrease in the touching capacitance as high as 15% under mechanical bending and the touch capacitance under bending conditions was not changed much similarly to the case of stretching.


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The capacitance change over time during finger-touching, obtained with the time period of 0.1 s for each point, showed an instant response to the touching with the capacitance change more than 15% (Figure 2(d)). The response and relax time obtained from the data in Figure 2(d) were about 1.3 s and 0.5 s, respectively. The observed difference in the response and relax time is attributed to the time difference between the movements of finger when touching or de-touching. Users usually move their finger faster when de-touching than touching the sensor while the fringe fields of capacitor instantly respond to these movements of finger. The optical transmittance of the transparent and stretchable capacitive touch sensor was shown in Figure 2(e) together with that of a single electrode on substrate. The optical transmittance of 78.6% for the device at 525-nm wavelength lower than that of the single electrode due to stacking of two electrodes on the top and bottom of substrates and formation of additional interfaces in between the layers. The capacitance of the sensor is expected to change during stretching with or without touching because both the area and thickness of the PU dielectric layer changed. The capacitance under stretching is given by the equation (1). C = ϵ ϵ

= C 1 + ε

(1) (2)

where ε is the applied strain in x direction, and is the Poisson’s ratio of dielectric and substrates as isotropic materials and C is the initial capacitance. In case of the same Poisson’s ratio of dielectric and substrates, capacitance increases in proportion to the applied strain ε and gauge factor (GF) has an ideal value of 1 as follows;

GF =

(3)


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(4)

=1 However, the touch sensor should be sensitive only to touch but not be sensitive to a large stretching strain to ensure stable operation of the sensor as an input device. When the capacitive strain gauge factor, ∆C/Coε, is too large, extraction of the sensor signals induced by touching under stretched conditions becomes difficult due to the large signal change caused by stretching. The mechanism of capacitance change for the touch sensor under stretching is illustrated in Figure 3. The sensitivity of the touch sensor to the strain can be controlled by adjusting the Poisson’s ratio of the dielectric materials compared to that of the substrate. In this work, waterbased PU was chosen as a dielectric layer and PDMS was chosen as a substrate; these have Poisson’s ratios of ~0.3524,25 and ~0.526, respectively, and were adopted to minimize the strain gauge factor of the touch sensor under stretched conditions. The thickness of the PDMS substrates (~500 µm) was much larger than that of thin dielectric (~10 µm). Therefore, the change in the in-plane area of the whole capacitor followed the deformation of PDMS substrates under stretching. For instance, under a 60% unidirectional tensile strain (along the x-axis), the PDMS substrate with a Poisson’s ratio of 0.5 shrinks by 30% perpendicular to the strain (along the y- and z-axis). Although the Poisson’s ratio of PU is ~0.35, the PU thin film sandwiched between top and bottom PDMS substrates should also shrink by 30% (green color), rather than by the 21% expected for a free-standing PU layer (represented by the blue color in the figure) based on its the initial shape (represented with gray color) because the PU follows the deformation of the PDMS substrate. Now, the PU dielectric layer has two boundary strain conditions: the 60% applied tensile strain (along the x-axis) and 30% compressive strain vertical to the straining direction (along the y-axis) from the top and bottom PDMS substrates. Based on


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a simple calculation using Hooke’s law27, the obtained shrinkage of the dielectric thickness was only 16.2%, rather than 30%, because over-shrinkage of the dielectric following the substrate along the y-axis restricted shrinking along the z-axis. In other words, the change in thickness for the dielectric was decreased by an increase in the change of width (vertical to the strain). Equation (1) had smaller numerator and bigger denominator and could not be expressed any more in a form as simple as the equation (2). Consequently, the theoretical value of the capacitive gauge factor for the structure of thin PU sandwiched in between two thick PDMS substrates became less than 1, calculated to be ~0.556. Considering the case of Poisson’s ratio is varied with respect to the applied strain range28, deformation of the dielectric should be integrated with respect to the strain (Figure S3 in Supporting Information). Surprisingly, until the Poisson’s ratio of the dielectric reaches the same value of substrate, 0.5, the deformation of dielectric thickness was not much different from the case of the constant value of Poisson’s ratio and the difference was expected not to be much more than 10%. Even assuming that the Poisson’s ratio increased rapidly and had value of 0.5 under strain, the effect of decreasing the total deformation of dielectric thickness and consequent decrease in the GF value was still valid. Details related to these calculations are provided in Supporting Information. To verify the result of theoretical calculation, finite element analysis (FEA) simulation was also conducted for the capacitor with a constant Poisson’s ratio of dielectric, 0.35 (Figure S4 in Supporting Information). The result of the FEA simulation was very close to the result of the theoretical calculation, which showed the change of dielectric thickness was 16.3% for the FEA simulation and 16.15% for the theoretical calculation. And the consequent GF values for the FEA simulation and theoretical calculation were evaluated as ~0.564 and ~0.556, respectively, under 60% of stretching strain. However, the experimental strain gauge factor of our device structure


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obtained from the slope in Figure 2(c) was only 0.10 to 0.13, which is much less than the theoretically calculated value of ~0.556. The experimental strain gauge factor of mutual capacitive touch sensors is also known to be in the range of 0.5 to 0.75, which is also lower than the theoretical value of 1.0 for an isotropic material5,6,21. The experimental strain gauge factors for stretchable capacitive strain or touch sensors are summarized in Table 1. As seen in the data, most other sensors utilizing Ecoflex or PDMS as dielectric layers show larger strain gauge factors than that of the transparent and stretchable touch sensor in this work. The fact that our experimental gauge factor of 0.10~0.13 is much lower than the expected theoretical value of 0.556 might be attributed to the electric fieldinduced strain12 or fringing field encountered in capacitors29. The electric field-induced strain could decrease the strain gauge factor due to the applied strain because the sample is strained right after the electric field across the electrodes is applied to the sample to measure its capacitance. Fringing fields are generally dominated by the dimensional ratio of width (or length)/thickness in the dielectric medium and are not negligible in our device, which uses fringing fields for capacitive touch sensing. The stain gauge factor can be less than the value expected from the capacitance inside the plates because capacitance from a fringing field is weakly dependent on dimensional changes in the dielectric compared to the capacitance from the field inside the parallel plates29. In contrast, the capacitance decreased by more than 20% by finger touching under stretching or non-stretching conditions. When touched, the fringing field was severely distorted, and the total capacitance decreased remarkably. Therefore, our stretchable and transparent capacitive touch sensor can distinguish the sensor signal by finger touch from the signal change by elongation strain up to 60%. A stretch-unresponsive touch sensing in our device was obtained for the sensor with a PU dielectric layer sandwiched between


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PDMS substrates. Using a dielectric with a relatively small Poisson’s ratio and a substrate with a larger Poisson’s ratio resulted in less shrinking of the dielectric thickness. This resulted in a decrease in the capacitance change, resulting in a decrease in the strain gauge factor. Further reduction in the strain responsivity of the device by other factors helped to provide a minimally stretch-sensitive stretchable capacitive touch sensor. An array of 5×5 stretchable touch sensors was also fabricated, and their touch sensing characteristics were measured. Figure 4(a) shows the fabrication process of a 5×5 array of stretchable and transparent capacitive touch sensors. Each pixel was 6 mm by 6 mm with line and space of electrodes of 3mm and 3mm, respectively. The total active area was 30 mm×30 mm. Details related to the experimental conditions for fabrication of the sensor array are provided in the Experimental Section. The same selective patterning process described above was used to pattern an array of stretchable electrodes. Selective patterning of stretchable electrodes was conducted by using an oxygen plasma treatment of a highly hydrophobic PDMS substrate through a PDMS shadow mask. After oxygen plasma treatment and removing the PDMS shadow mask, it is easy to pattern the stretchable electrodes by simple dropping and spinning of AgNWs and rGO dispersions. Then, a PU dispersion was spin-coated on the whole surface of the substrate with a selectively formed electrode pattern of AgNWs/rGO. Other methods like screen printing6,21 have been used to repeat the patterning process to form each thin film pattern. This is not cost-effective and it can also result in poor alignment on stretchable substrates. Using our surface energy modification method, the whole fabrication process was simplified. In addition, we prevented alignment failure, and could easily obtain complex electrode patterns and shapes. After hot-pressing the two substrates, electrical contacts were formed by using EGaIn and Au textiles for touch sensing measurements.


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Figure 4(b) shows the optical image of a 5×5 array of stretchable transparent capacitive touch sensors. The optical image of the sensor array indicates that line patterns of stretchable and transparent electrodes using solution processes were successful. Figure 4(c) shows a crosssectional FE-SEM image of a capacitive array sensor. Upper and lower electrodes of AgNWs/rGO can be clearly observed with a thickness of ~7 µm. Conducting networks of AgNWs/rGO seem to be well embedded into the PU dielectric layer in the AgNWs/rGO/PU structure using just coating and curing processes. Figure 4(d) and (e) show the relative capacitance change value ((C0-∆C)/C0) of the capacitive touch sensor array as the diagonal points were touched. When pixel (1,1) was touched, the capacitance decreased to 60% of the initial value around pixel (1,1), while the capacitance decreased only to 74% of the initial value around the opposite diagonal pixel (5,5) (Figure 4(d)). In contrast, when pixel (5,5) was touched, the capacitance of pixel (5,5) decreased to 54% of the initial value, while it decreased to 84% of the initial value for pixel (1,1) (Figure 4(e)). The array of capacitive sensors showed touch sensing ability and showed differences in the capacitance changes with respect to the distance from the touching points. The results indicate that the array touch sensor can distinguish the touching point as a capacitive touch sensor. The non-uniformity of the capacitance of the array is attributed to the variations in the thickness of the PU dielectric layers and variations in pattern sizes due to the knife-cut PDMS shadow mask. A more precise dielectric layer thickness might be obtained by carefully adjusting the volume of the PU solution for each electrode. The pattern size of the PDMS shadow mask can be carefully controlled by laser cutting, 3D printing, etc., to improve the uniformity of the sensor characteristics.

3. CONCLUSION


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Here, we demonstrated a stretch-unresponsive stretchable and transparent touch sensor by using a thin PU dielectric sandwiched between two stretchable and transparent electrode lines on PDMS substrates. AgNWs/rGO nanocomposites embedded into a PU dielectric layer coated on PDMS substrates showed high conductivity, stretchability and optical transparency. The AgNWs/rGO electrodes embedded into a PU dielectric layer showed no significant change in conductance under strain up to 50%. The addition of rGO nanoflakes on AgNWs enhanced both the conductance and stretchability of the AgNW conducting network. The different Poisson’s ratios of the PDMS substrate and PU dielectric layer helped to distinguish between the stretching strain sensitivity and the touching sensitivity of the sensor. The fabricated stretchable and transparent touch sensor was much more sensitive to finger touch rather than elongation strain due to the proper selection of materials with different Poisson’s ratios for capacitive dielectrics and substrates. An array of 5×5 stretchable transparent capacitive touch sensors based on patterned AgNWs/rGO lines embedded in PU dielectrics on PDMS substrates was demonstrated. This array of capacitive sensors showed touch-sensing capability with differences in capacitance with respect to the distance from the touching points. Capacitive touch sensors based on these AgNWs/rGO electrodes with the selective patterning method are a very promising technology that is simple and cost-effective. These stretchable and transparent touch sensors with stretchunresponsive touch sensing capability are promising for applications in wearable electronics and human-machine interfaces requiring both conformal and compliant mechanical properties and optical transparency.

4. EXPERIMENTAL SECTION


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Materials. PDMS substrates were prepared by mixing the “base” and the “curing agent”, which were purchased from Dow Corning Co., Ltd., USA (“base” Sylgard 184/ “curing agent”, respectively) at a ratio of 10:1. After vacuum degassing, the liquid mixture was thermally cured at 80°C for 4hrs to form a cross-linked PDMS substrate. AgNWs dispersed in isopropyl alcohol (IPA) were purchased from Nanopyxis Co., Ltd., Korea. The rGO solution was synthesized by reducing a GO dispersed solution. GO nanosheets dispersed in N,N’-dimethylacetamide (DMAC) were reduced

by

a

two-step

in-situ

reduction technique,

which

consisted

of

a

phenylhydrazine reduction (2 µL hydrazine per 1mL of dispersed GO in DMAC with a concentration 5mg/mL) and a subsequent thermal reduction at 150 °C for 4 hrs under vacuum33. Reduction of GO to rGO was confirmed by Raman spectroscopy as shown in Figure S5 in Supporting Information. A water-based aliphatic polyester PU dispersion was purchased from Alberdingk Boley (Alberdingk U3251) and was diluted to 10 wt% with DI water to reduce the thickness of the dielectric layer. Fabrication of single stretchable and transparent capacitive touch sensor device. A solution of dispersed AgNWs was spin-coated with 1000 rpm for 20 s on PDMS substrates pretreated with oxygen plasma at 500 W for 5 min in a microwave plasma reactor. AgNW thin films on PDMS substrates were dried at 60℃ for 1 min and annealed at 120℃ for 10 min in a nitrogen (N2) environment. After spin-coating and annealing of the AgNW thin film, the rGO solution was spin-coated on top of the AgNW-coated PDMS substrates. The rGO solution was readily reduced from GO by hydrazine. After coating, the films were dried at 60℃ for 10 min, and annealed at 170℃ for 4 hrs in an N2 environment. Then, the PU dispersion was spin-coated on the AgNWs/rGO films at 4000 rpm for 20 s. To prevent insulating PU from coating the electrical contact areas, pieces of PDMS were used as coating masks to cover the contact area during PU


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coating. Then, PU films were dried and annealed at 60℃ for 10 min and 150 ℃ for 1 hr in an N2 environment. To form a capacitor, we hot-pressed two substrates with the PU surfaces facing each other at 1000 psi of pressure and at 90℃ for 4 hrs. After attaching the two substrates on a single capacitive touch sensor, EGaIn was applied and Au textile was attached on the contact area as an electrical contact. Fabrication of stretchable and transparent capacitive touch sensor array. PDMS substrates have a highly hydrophobic surface. Selective patterning was conducted by using oxygen plasma treatment via a PDMS shadow mask. Oxygen plasma treatment of 500 W for 5 min in a microwave plasma reactor converted the hydrophobic surface of PDMS to a hydrophilic surface. After removing the PDMS shadow mask, solutions of AgNW, rGO and PU were coated in sequence and were dried under the same conditions used for single capacitive touch sensors without further plasma treatment. An optical image of the electrode array pattern after coating the AgNW and rGO solutions is shown in Figure S6 in Supporting Information. Thin films were formed only on the plasma-treated region due to the surface energy difference between the plasma-treated region and non-treated region. The electrical contact region was covered by a PDMS mask during spin-coating of the PU solution. Finally, the two substrates were hot-pressed at 90℃ for 4 hrs with the PU surface facing each other. Electrical contacts were formed for measurements by using EGaIn and Au textiles. Measurements. Measurements of sheet resistance for thin films (AgNWs, AgNWs/PU and AgNWs/rGO film) were conducted by using a semiconductor parameter analyzer (B1500A, Agilent Technologies) under stretching conditions provided by the custom-built cyclic stretching system. Au textiles (Solueta Co. Ltd., silverized nylon/spandex knit SMP 130) were


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attached by using EGaIn on each end side of the sample to reduce the damage on the thin films on the elastomeric substrate from sharp tips of measurement system. Then, the sample was loaded and tightly held on the stretching system which has the measurement tips inside the holder as show in Figure S7 in Supporting Information. The electrical conductance was measured while stepwise strains were applied to the sample. The same semiconductor parameter analyzer was used for measurements of capacitances for single capacitive touch sensor and an array of capacitive touch sensor. The top-view image of the AgNWs/rGO thin film and cross-sectional image of the capacitive touch sensor were measured by FE-SEM (JEOL JSM-3500F). Simulation. We constructed a finite element model to calculate the change of thickness and width of a capacitor with 1.5 cm × 1.5 cm of area when stretched. The capacitor consisted of two PDMS substrates and a PU dielectric with thickness of 0.5 mm and 10 μm, respectively. Both PDMS and PU domains were discretized using 8-node hexahedral elements where the displacement-pressure mixed finite element was used for PDMS layers to consider their incompressibility. The finite element model consisted of 48,000 elements and 52,111 nodes in total. We performed nonlinear analysis for this model by applying the tensile strain of 60% (or +0.9 cm) in one direction using a commercial finite element program, ADINA.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.


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Scheme of fabrication process for single capacitive touch sensor, relative capacitance change under bending conditions, detailed calculation of strain gauge factor, finite element analysis, Raman spectra, optical images of array pattern during fabrication, optical images of custom-built stretching system (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Note The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the Basic Science Research Program (2016R1A2A1A05005423) through the National Research Foundation (NRF), funded by the Ministry of Science, ICT & Future Planning.

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Figure 1. Characterization of stretchable and transparent electrode materials. (a) FE-SEM image of AgNWs imbedded into PU thin film formed by spin-coating of a PU dispersion on AgNWs. White arrows indicate the part of AgNWs upward to the surface of the PU and red arrows downward deep inside the PU. (b) Comparison of sheet resistances for AgNWs, AgNWs/PU, and AgNWs/rGO/PU formed on PDMS substrate under stretching strain up to 50%. (c) Top-view FE-SEM image of AgNWs/rGO film showing AgNWs connected with rGO nanoflakes at the junctions. (d) Conductance of AgNWs/rGO/PU film under cyclic stretching test at 30% stretching.


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Figure 2. Structure of transparent and stretchable touch sensors and touch sensing under stretching. (a) Schematic of transparent and stretchable capacitive sensor. (b) Touch sensing mechanism of (mutual) capacitive touch sensor. Non-touching state (left) and touching state (right). The fringing electric field between top and bottom electrodes is partially distributed to the touching finger in the touching state. (c) Capacitance change in single capacitive touch sensor under stretching strain with (red) / without (black) touching (inset: photographs of single capacitive touch sensor with 0% (left) and 60% (right) of stretching during stretching test). Capacitance change in the device before and after finger touch is nearly unchanged with and without stretching. (d) Capacitance change (C/Co) over time under finger-touching. (e) Optical transmittance spectra of single stretchable AgNW/rGO/PU electrode and stretchable capacitive touch sensor with two AgNW/rGO/PU electrodes.


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Figure 3. Mechanism of capacitance change of touch sensor under applied stretching strain. The device is stretched in the x-direction and the shapes of PU without and with top and bottom PDMS substrates are represented with blue and green color, respectively.


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Figure 4. Stretchable and transparent touch sensor array. (a) Fabrication process of 5×5 array of stretchable capacitive touch sensors. (b) Optical image of 5×5 array of stretchable transparent capacitive touch sensor. (c) Cross-sectional FE-SEM image of capacitive array sensor. (d, e) The relative capacitance change ((C0-∆C)/C0) of capacitive touch sensor array under finger touching at pixel (1,1) and pixel (5,5), respectively.


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Table 1. Summary of strain gauge factors in recently reported stretchable capacitive sensors Reference

Materials

Strain gauge factor

Capacitance change by finger touch −∆/ )

(Substrate/electrode/dielectric) [1]

PDMS/CNTs/Ecoflex

0.4

-

[15]

PU/AgNWs/3M Scotch 924 ATG Tape

0.5

~7%

[6]

PDMS/AgNWs/Ecoflex

0.7

15~20%

[30]

PDMS/Au/PDMS

0.75

25%

[31]

PDMS/CNTs/Dragon skin elastomer

0.97

-

[16]

Silicone/CNTs/silicone

0.99

-

[32]

PDMS/AgNWs/PDMS

1

-

This work

PDMS/AgNWs-rGO/PU

0.1

21.9~23.6%


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Table of Contents Graphic


30

Stretchable, Transparent, and Stretch-Unresponsive Capacitive Touch

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