Highly Sensitive and Flexible StrainâPressure Sensors with Cracked

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Applications of Polymer, Composite, and Coating Materials

Highly sensitive and flexible strain-pressure sensor with cracked paddy shaped MoS/graphene foam/Ecoflex hybrid nanostructures 2

Seong Jun Kim, Shuvra Mondal, Bok Ki Min, and Choon-Gi Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11233 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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ACS Applied Materials & Interfaces

Highly sensitive and flexible strain-pressure sensor with cracked paddy shaped MoS2/graphene foam/Ecoflex hybrid nanostructures

Seong Jun Kim,† Shuvra Mondal,†, ‡ Bok Ki Min,† and Choon-Gi Choi†,‡,*

†

Graphene Research Lab., Emerging Devices Research Group, Electronics and

Telecommunications Research Institute (ETRI), Daejeon 34129, Republic of Korea ‡

School of ETRI (ICT-Advanced Device Technology), University of Science and Technology (UST), Daejeon 34113, Republic of Korea *

Corresponding author E-mail: [email protected]

*

corresponding author. Tel.: +82-42-860-6834. Fax: +82-42-860-5211. E-mail address: [email protected] (Choon-Gi Choi). ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Three-dimensional graphene porous networks (GPNs) have received considerable attention as a nano-material for wearable touch sensor applications due to their outstanding electrical conductivity and mechanical stability. Herein, we demonstrate a strain-pressure sensor with high sensitivity and durability by combining molybdenum disulfide (MoS2) and Ecoflex with a GPN. The planar sheets of MoS2 bonded to the GPN were conformally arranged with a crack-paddy shape, and the MoS2 nano-flakes were formed on the planar sheet. The size and density of the MoS2 nano-flakes was gradually increased by raising the concentration of (NH4)2MoS4. We found that this conformal nanostructure of MoS2 on the GPN surface can produce improved the resistance variation against external strain and pressure. Consequently, our MoS2/GPN/Ecoflex sensor exhibited noticeably improved sensitivity compared to previously reported GPN/PDMS sensors in a pressure test due to the existence of the conformal planar sheet of MoS2. In particular, the MoS2/GPN/Ecoflex sensor showed a high sensitivity of 6.06 kPa-1 at (NH4)2MoS4 content of 1.25 wt%. At the same time, it displayed excellent durability even under repeated loading-unloading pressure and bending over 4000 cycles. When the sensor was attached on a human temple and neck, it worked correctly as a drowsiness detector in response to motion signals such as neck bending and eye blinking. Finally, a 3 × 3 tactile sensor array showed precise touch sensing capability with complete isolation of electrodes from each other for application to touch electronics applications.

KEYWORDS: Wearable sensor, MoS2/GPN/Ecoflex hybrid nanostructures, Strain-pressure sensor, Human-motion detection, Flexible tactile sensor

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INTRODUCTION Flexible strain-pressure sensors that are attachable on human skins or body has extended their applications of conventional pressure sensors to a much wider range such as monitoring of physiological signals, electronic skin, human–machine interactions, artificial limb implants. 1-4

To be applicable in such broader spectrum, a wearable strain-pressure sensor should be

able to accurately detect subtle body signals from external stimuli in an extensive sensing range from pulse signals to knee joint bending. Furthermore, they must exhibit high reliability and reproducibility after repeated mechanical strain. Carbon-based polymer composites such as PC/CNT, PU/Cotton/CNT, CNT/Ecoflex etc. combined with electrical conductivity and mechanical flexibility have been a popular choice for fabricating such sensors.5-7 Due to their superior mechanical, electrical properties, and unique twodimensional (2D) structure where each atom can potentially contribute to sensing response, graphene-based materials have attracted intense interest in pressure sensors. However, strong van der Waals interactions and high inter-sheet junction contact resistance between graphene sheets critically suppress the high conductivity and mechanical strength of individual graphene sheets.8,9 To solve this issue recently, three-dimensional graphene foam (GF) having a wide specific surface area and remarkable electrical conductivity has garnered significant interest as a sensing material due to its excellent elasticity with a rapid rate of recovery.10-20 Most graphene foam-based strain and pressure sensors are a piezoresistive type that convert mechanical deformation to resistance signals, and they have been extensively explored due to their simplicity of fabrication, low cost, and ease of signal processing.21-23 Y. A. Samad et. al. reported a two-step process using graphene oxide (GO) coated polyurethane (PU) foam for a low and high strain-pressure sensor.24 Y.R. Jeong et. al. demonstrated a

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strain sensor with high stretchability (over 70%), durability (10,000 cycles), and sensitivity (Gauge factor, GF: 15 to 29) using a fragmentized graphene foam (FGF)/PDMS composite.25 Nevertheless, the GF based pressure sensor still exhibited a sensitivity of less than 1 kPa-1 compared to conventional two-dimensional material-based films.14,20,22 Meanwhile, as a nontoxic and environmentally friendly, bandgap-tunable two-dimensional semiconducting material MoS2 has demonstrated appealing properties including a large surface-to-volume ratio, excellent piezoresistivity, chemical stability, and high mechanical strength and flexibility, making it a good candidate for strain and pressure sensors.26-31 M. Park et. al. demonstrated ultrathin conformal MoS2 coupled with a graphene electrode to create a high gauge factor and reproducible tactile sensor.32 W. Park et. al. reported a piezoelectric PVDFTrFE/MoS2-based pressure sensitive touch senor with reliable switching behavior.33 Nevertheless, few studies related to MoS2-based strain and pressure sensors have been reported. In this study, we demonstrate a cracked paddy-shaped MoS2/graphene porous network (GPN) infiltrated Ecoflex hybrid nanostructure as a wearable strain-pressure sensor. Owing to the conformal MoS2 planer sheets with cracked paddy shape attached to the GPN surface, our sensor exhibited high sensitivity of 6.06 kPa-1 for pressure and a gauge factor (GF) of 24.1 for bending strain. In addition, Ecoflex is eco-friendly and harmless to the human body, thus Ecoflex-encapsulated MoS2 is considered a suitable candidate for sensor application to human body. The fabricated sensors showed potential in practical applications such as a human motion sensor to detect drowsiness and an array type tactile sensor.

RESULTS AND DISCUSSION The morphology of pure nickel Ni foam, graphene porous network (GPN) on Ni foam, and 4


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GPN coated with different concentrations of (NH4)2MoS4 (0.2, 0.5 and 1.25 wt%) on Ni foam, respectively (Figure 1a‒e), was characterized by scanning electron microscopy (SEM). Figure 1a shows pure Ni foam with a skeleton having large pore sizes of 100-200 µm. The synthesis of graphene on Ni foam by a thermal chemical vapor deposition (TCVD) process, compared to the solution process, is a useful way to grow a uniformly distributed multilayer graphene over the whole surface of Ni foam. The magnified SEM image in Figure 1b clearly exhibits that graphene was uniformly deposited on nonplanar nickel surface with a smooth surface. Continuously, MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4) planar sheets coated onto graphene on Ni foam by a dipping process formed a cracked paddy structure on the graphene surface after an annealing process at 600 °C, as shown in Figure 1c‒e. Furthermore, as the concentration of (NH4)2MoS4 was increased from 0.2 to 1.25 wt%, the grain size and density of MoS2 nano-flakes on MoS2 planar sheets attached to the GPN gradually increased (see inset of Figure 1c‒e). This MoS2/GPN nanostructure can significantly contribute to improving the sensitivity of the pressure sensor due to the continuous arrangement of conformal MoS2 planar sheets, which forms the cracked paddy field. Figure 1f shows photographs of pure Ni foam, GPN on Ni foam, and a GPN with various (NH4)2MoS4 content (0.2, 0.5 and 1.25 wt%) on Ni foam. After graphene was synthesized on Ni foam, the color of the Ni foam changed from gray to black. We additionally performed combined studies including scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS, Bruker) to identify chemical element composition of MoS2/GPN attached on Ni foam and their structural features. As shown in Figure 1g-j and Figure S1, the Mo and S were only observed in MoS2 planar sheets and MoS2 nano-flakes, while carbon (C) was only showed in exposed areas of GPN without MoS2 planar sheets. From these results, we propose the 5



following formation mechanism for the cracked paddy structure of MoS2 on graphene. The (NH4)2MoS4 was coated on the surface of porous graphene, and MoS2 was formed by desulfurization at a temperature of 600 °C. Herein, we believe that MoS2 is synthesized with cracked paddy structure after the synthesis and cooling process due to graphene with negative thermal expansion coefficient. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical structure and arrangement of the MoS2 contained in MoS2/GPN on Ni foam. Figure S2a displays XPS core level spectra of the Mo 3d and S 2p for MoS2 (1.25 wt% (NH4)2MoS4). From the curve fitting of the Mo 3d core level spectrum, Mo4+3d5/2, Mo4+3d3/2, and S 2s were observed at binding energies of 229.4, 232.5 and 226 eV, respectively. In addition, the peak related to MoO3 was weakly observed at a binding energy of 235.6 eV. In case of S 2p core level spectrum, the S 2p3/2 and S 2p1/2 peaks corresponding to MoS2 appear at binding energies of 162.1 and 163.3 eV, respectively. From these results, we confirmed that MoS2 is physically attached to the GPN surface without chemical deformation. Furthermore, the increase of MoS2 content was confirmed by the atomic ratio of Mo/C with increasing the (NH4)2MoS4 content (Figure S2b). Raman spectroscopy with a laser excitation wavelength of 532 nm was used to characterize structural features of the graphene porous network (GPN) and MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4)/GPN composites before and after encapsulating with Ecoflex and Ni etching, as shown in Figure 2. Figure 2a shows the Raman spectra for GPN grown on Ni foam. In the case of the GPN, the two predominant peaks related to the G- and 2D- band of graphene appeared at 1585 and 2715 cm-1, respectively.34 Also, the intensity ratio of IG/I2D was greater than 2, indicating that multilayer graphene was grown on the Ni foam.35,36 After infiltrating Ecoflex into the GPN and removing Ni, GPN/Ecoflex exhibited similar peak positions 6


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compared to pure GPN/Ni foam, with the exception of additional polymer peaks at 499, 708, 2912, and 2971 cm-1 (Figure 2e). Figure 2b‒d shows the Raman spectrum of MoS2/GPN on Ni foam with different concentrations (0.2, 0.5 and 1.25 wt%) of (NH4)2MoS4. Two prominent peaks associated with the in-plane E12g and out of plane A1g phonon modes of MoS2 were observed in the range of 380‒420 cm-1 with graphene peaks.37,38 In addition, the difference between A1g and E12g of MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4) was over 25 cm-1, indicating multilayer MoS2. After the MoS2/GPN hybrid nanostructures were encapsulated by Ecoflex and subsequently etching away Ni, the Raman spectrum was observed and results were similar to those obtained before covering GPN with Ecoflex, with the exception of additional polymer peaks (Figure 2f‒h). From these results, we confirmed that the GPN/Ecoflex and MoS2/GPN/Ecoflex hybrid nanostructures were well-formed without any structural deformation or noticeable damage that could degrade the device performance. Pressure tests were performed to investigate the sensing performance of the GPN/Ecoflex and the MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4)/GPN/Ecoflex devices at various pressures, respectively, as shown in Figure 3. A tapping machine (Force Gauge Model M2-10) was employed for the pressure test, as shown in Figure 3a. Figure 3b shows the relative resistance variation

for

the

GPN/Ecoflex

and

the

MoS2

(0.2,

0.5

and

1.25

wt%

(NH4)2MoS4)/GPN/Ecoflex pressure sensors under applied pressures ranging from 0.6 to 25.4 kPa. Interestingly, the relative resistance variation of the MoS2/GPN/Ecoflex sensors was remarkably higher than that of the GPN/Ecoflex sensor. Moreover, as the concentration of (NH4)2MoS4 in MoS2/GPN composite was increased, the sensitivity of the device was

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considerably enhanced. Here, the slope of the curve is the pressure sensitivity (S) and can be expressed by the following equation: S=δ(∆R/R0)/δP

(1)

where ∆R, R0, and P represent the change in resistance, the initial resistance, and applied pressure, respectively. From the above equation, we can obtained the pressure sensitivity of GPN/Ecoflex and the MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor. From the Figure 3b, we have confirmed that linear response for resistance variation of MoS2/GPN/Ecoflex and GPN/Ecoflex is separated by 3 slopes and 1 slope, respectively. This phenomenon can be explained by the fact that when the pressure exceeds a certain level, the distance between the adjacent MoS2 planar sheets becomes close to each other, thereby additional conduction path is formed resulting in a decrease in resistance. Subsequently, as the applied pressure further increases, the conductive pathway no longer increases and becomes saturated.34 In addition, MoS2 covers a part of the graphene instead of the entire graphene so that reversibility can be improved by suppressing the peeling of interface between graphene and MoS2 even under large strain or pressure. The pressure sensitivity of four different pressure sensors can be classified according to three linear resistance variation regions, as shown in Figure 3c: (1) at low pressure of 0.6‒7.6 kPa, the slopes for the GPN/Ecoflex and the MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4)/GPN/Ecoflex pressure sensors were 0.069, 0.41, 2.45 and 3.28 kPa-1, respectively; (2) at intermediate pressure of 7.6‒15.2 kPa, the slopes for the GPN/Ecoflex and the MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4)/GPN/Ecoflex pressure sensors corresponded to 0.06, 0.58, 2.74 and 6.06 kPa-1, respectively; and (3) at high pressure 15.2‒25.4 kPa, the slopes for the GPN/Ecoflex and the MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4)/GPN/Ecoflex pressure sensors were 0.055, 0.49, 8


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2.31

and

3.47

kPa-1,

respectively.

In

the

case

of

the

MoS2 (1.25

wt%

(NH4)2MoS4)/GPN/Ecoflex pressure sensor, the pressure sensitivity was much higher than that of the other three devices, but the linearity was found to be slightly lower. Continuously, we evaluated the sensing performance according to the applied pressure by increasing the concentration of (NH4)2MoS4 solution from 1.25 to 2.5 wt%. The sensitivity of MoS2 (2.5 wt% (NH4)2MoS4)/GPN/Ecoflex sensor was lower than that of the sensor with the concentration of 1.25 wt% (NH4)2MoS4, as shown in Figure S4a. This result is due to the fact that 2.5 wt% (NH4)2MoS4 is not well dispersed in ethylene glycol, so MoS2 completely covers the GPN surface instead of forming a cracked paddy pattern to improve the sensing performance after the annealing process (in a SEM image of Figure S4b). Figure 3d exhibits the timedependence relative resistance variation of the MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex pressure sensor under various pressures. When an external force is applied to the pressure sensor, the corresponding resistance reduces because of improved physical contact between adjacent graphene, resulting in enhanced conductive pathways.39-41 As a result, the relative variation in resistance (∆R/R0) of the sensor increases to the negative direction in proportion to the applied pressure. Furthermore, the sensor showed excellent reproducibility during repeated loading-unloading at various pressure ranges. The durability of the MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex was evaluated by real-time monitoring for 1,000 s and pressure cycles from 1 to 4,000 under cyclic pressing-releasing of pressure of 5.08 kPa, respectively, as shown in Figures 3e,f. As a result, the sensor showed long-term stability characteristics without degradation upon repeated loading-unloading pressure, indicating excellent durability for real life applications. As well as, MoS2/GPN/Ecoflex sensors exhibited a high sensitivity and durability from comparing of main parameters with other conventional composite

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pressure sensors (Table S1).7,14,22,42,43 Figure 4a‒d shows the sensing performance of the MoS2 (1.25

wt % (NH4)2MoS4)/GPN/ Ecoflex strain-pressure sensor for changes in tensile

and bending strain. The MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor was measured at the tensile speed of 0.55 mm/min using the tensile testing machine. As shown in Figure 4b, when the tensile strain was increased from 0 to 22% and then decreased, the time-relative resistance change curves according to the applied strain exhibited excellent restoring ability, displaying near symmetry. Generally, the bending strain can be obtained from bending radius and calculated by the following equation: ε = d/2r, ε = -d/2r, where ε is the bending strain, d is the thickness of the sample, and r is the bending radius.41,44 Herein, the thickness (d) of the sample is 1.3 mm, and the bending radius (r) for each bending strain are shown in Figure S3a. The bending strain obtained from the above equation was converted into a value of 100percent. Figure 4c describes the relative resistance variation of the sensor as a function of bending strain from 12.6 up to 50% using a bending machine. When the bending strain increases, the resistance increases due to the increase in the length of the MoS2 planer sheets combined with graphene foam. At the same time, the distance between the adjacent cracked MoS2 sheets also is farther away resulting in decrease of the conductive pathway. Consequently, the relative resistance variation continues to increase linearly in proportion to the bending strain. Here, the slope of the graph is a parameter indicating the performance of the sensor as a gauge factor (GF), which can be expressed by the following equation. Gauge factor (GF) = (∆R/R0)/ε

(2)

where R, R0, and ε represent the change in resistance, resistance at 0% strain, and applied strain, respectively. From the bending test (Figure 4c), we confirmed that the MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor exhibited both excellent linearity and a high gauge factor 10


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of 24.1. Figure 4d displays the relative resistance response of bending-releasing cycles of the MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor as a function of the applied bending strain. The response signals of the device maintained stable performance despite repeating bending-releasing cycles during the increase of bending strain. Furthermore, we have measured a variation in resistance for repeated bending strain to evaluate the long-term stability of the MoS2 (1.25 wt% (NH4)2MoS4))/GPN/Ecoflex sensor. As a result, MoS2 (1.25 wt% (NH4)2MoS4))/GPN/Ecoflex sensor exhibited high durability with a constant value of about 8 at relative resistance change even after 4,000 bending cycles under 33.3% bending strain as shown in Figure S3b. The MoS2/GPN/Ecoflex based strain-pressure sensor exhibited light weight, superior mechanical flexibility, and good stretchability, as shown in Figure 4e. In particular, Ecoflex is chemically harmless to human skin, and can be used in practical applications to easily detect human motion signals by attaching it to a human body. In this study, we demonstrated various human motion movements such as finger bending, eye blinking and neck movement using the MoS2/GPN/Ecoflex sensor, which was attached on a human finger, temple and neck, respectively. Metallic wires were connected to both ends of the sensor. Figure 4f shows the change in relative resistance in response to repeated bending and relaxation of the sensor that was attached on a human finger. When the index finger was bent, the relative resistance change of the MoS2 (1.25wt% (NH4)2MoS4)/GPN/Ecoflex sensor was about 80 in the positive direction. Additionally, the sensor was attached on a human temple to detect eye blinking response. As the eye blinking and relaxation were repeated, the relative resistance change was almost 1 (Figure 4g). Finally, the sensor was attached on a human neck to observe response signals corresponding to neck bending. As a result, a relative resistance variation of about seven appeared for several bending repetitions (Figure 4h). This

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work demonstrated that we can effectively detect signals related to diverse human movements such as finger bending, eye blinking and neck bending based on various resistance changes due to different compressive and tensile strain effects. In the near future, we expect that this technology will be used to calibrate any posture during exercise or to prevent fatal accidents caused by driving while drowsy. As shown in Figure 5a, a 3 × 3 tactile sensor array based on the MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex was prepared for application to artificial electronic skin. First, copper tape was placed on Kapton tape to serve as a bottom electrode. Nine MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensors of 1 × 1 cm2 dimension were then placed on the copper electrode. For separation from the top electrode, an insulating layer was used in the vertical direction of the bottom electrode, as shown in Figure 5a(iii). Finally, after aligning the Kapton tape with the top electrode to the position of the bottom electrode containing the sensor and the insulating layer, a flexible 3 × 3 tactile sensor array was completed by overlapping two sides, as shown in Figure 5a(v). Figure 5b shows a photo-image of the 3 × 3 tactile sensor array, where rows and column are indicated as 1, 2, 3 and a, b, c, respectively. The sensing performance of the 3 × 3 tactile sensor array based on MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex was evaluated by the relative variation of resistance with applied pressure. As shown in Figures 5c‒d and Movie S1, in the case where the I-V instrument was connected to a‒3 (or b‒1) as an electrode position when pressing and relaxation of a human index finger was repeated in the position of a‒3 (or b‒1), a relative resistance change of 12 (or 15) was exhibited in the negative direction with a stable resistance response. Meanwhile, when the electrode was connected to b‒2, pressure was applied sequentially on each location, as shown in Figure 5e and Movie S2. As a result, relative resistance variation appeared only 12


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at the position of b‒2 and the resistance remained constant for other positions, confirming complete isolation of the electrodes. Figure 5f shows the distribution of the relative resistance change using a bar graph when the electrode was connected to b‒2. A relative resistance change of about eight is exhibited in the negative direction at position b‒2 corresponding to Figure 5e. This precise switching sensing capability of the 3 × 3 tactile sensor array demonstrates its potential to be applied in human-mechanic interfaces, wearable skin electronics, touch sensors and touch screens, etc.

CONCLUSION In summary, we demonstrated a cracked paddy shaped MoS2/GPN/Ecoflex hybrid nanostructure to enhance the pressure sensitivity and flexibility for human motion detection and tactile sensor application. The size and density of nano-flakes formed on the MoS2 planar sheet increased as the concentration of (NH4)2MoS4 was increased from 0.2 to 1.25 wt%. As a result, the MoS2 (1.25wt% (NH4)2MoS4)/GPN/Ecoflex sensor exhibited higher device performance compared to that of the MoS2 (0.2 and 0.5 wt% (NH4)2MoS4)/GPN/Ecoflex sensors and the GPN/Ecoflex sensor in the pressure test. The MoS2/GPN/Ecoflex sensor showed stable characteristics without deterioration of device performance even under repeated pressing-releasing over 4,000 cycles. Furthermore, it showed a high gauge factor (24.1) and outstanding resilience in bending and tensile tests. Finally, the MoS2/GPN/Ecoflex sensor attached to a human body generated distinguishable resistance characteristics corresponding to various human motion signals, including finger bending, eye blinking and neck movement. In addition, the MoS2/GPN/Ecoflex sensor with conformal planar sheet of MoS2 demonstrated excellent switching response in the 3 × 3 tactile sensor array. We believe 13



that this study will invigorate future industrial and research fields related to wearable touch sensor applications.

METHODS Synthesis of Graphene Porous Network. Ni foam was cleaned with 40 ml of HCL (35‒ 40%) and 150 ml of DI water, and used as a template for graphene porous network (GPN) synthesis. The Ni foam was heated from room temperature to 1,000 °C inside a thermal chemical vapor deposition (TCVD) reactor. H2 (20 sccm) gas was introduced at a pressure of 9.5 mTorr for 60 min. When the growth process was performed in a 6 inch quartz tube reactor at the target temperature, CH4 (80 sccm) was introduced as a carbon feedback with H2 (20 sccm) for 30 min. Afterwards, the graphene porous network (GPN) grown on Ni foam was cooled to room temperature. Growth of MoS2 layers on GPN. The two-step thermal decomposition process using solution-processed (NH4)2MoS4 powder (99.95%, Acros Organics) to form MoS2 nano-flakes on the GPN is as follows. (NH4)2MoS4 was mixed with ethylene glycol to make a (NH4)2MoS4 solution having concentrations of 0.2, 0.5 and 1.25 wt%. Thereafter, the mixture was stirred at room temperature for 30 min using a magnetic bar. TCVD synthesized GPN on Ni foams was then dipped in (NH4)2MoS4 solution (0.2, 0.5 and 1.25 wt%) and dried in an oven at 90 °C for 1h. (NH4)2MoS4 solution coated GPN on Ni foams was then annealed at 280 °C by introducing Ar at pressure of 1.8 Torr for 30 min and subsequently annealed at 600 °C under an Ar (150 sccm) flow for 30 min to synthesize MoS2 nano-flakes on the GPN. 14


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Synthesis of MoS2/GPN/Ecoflex composite. Ecoflex rubbers (EcoflexTM 00-30) containing Part A and B were mixed with a ratio of 1:1 in a mixing container. After enough stirring of Part A and B, the MoS2/GPN on Ni foam was immersed in the Ecoflex solution and then cured at 80 ° C for 2 hours. Next, the MoS2/GPN infiltrated Ecoflex was immersed in a FeCl3 solution to etch the Ni foam as a template for 12 hours at room temperature. After rinsing several times in DI water, the MoS2/GPN/Ecoflex composite was completed.

Conflict of Interest: The authors declare no competing financial interest.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional data including SEM image and the corresponding EDS elemental mapping images of MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex, XPS core level spectra for Mo 3d and S 2p of MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor; the atomic ratio of Mo/C as a function of (NH4)2MoS4 content, various parameters such as bending radius, thickness of sample, and bending strain for bending test of MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor; durability test of the MoS2 (1.25 wt% (NH4)2MoS4))/GPN/Ecoflex strain sensor under repeated bending cycles from 1 to 4,000 with an applied bending strain of 33.3%; comparison of relative resistance variation for MoS2/GPN/Ecoflex sensors with the concentration of (NH4)2MoS4 1.25 and 2.5 wt%, respectively, SEM image of of MoS2 (2.5 wt% (NH4)2MoS4)/GPN on Ni foam (Figure S1-S4). Table data for comparison of sensing 15



performance between MoS2/GPN/Ecoflex composite- and other conventional composite pressure sensors. (Table S1). Movie files for the sensing performance of the 3 × 3 tactile sensor array based on MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex (Movie S1, S2).

Acknowledgement. This work was supported by Electronics and Telecommunications Research Institute (ETRI) grant (No: 18ZB1140) funded by the Korean government and the nuclear R&D program (No: 20181510102340) supported by the Ministry of Trade, Industry & Energy of the Korean government.

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FIGURE CAPTIONS

Figure 1. Representative SEM images of (a) pure Ni foam and (b) GPN on Ni foam. SEM images of MoS2/GPN with different concentrations of (NH4)2MoS4 on Ni foams: (c) 0.2, (d) 0.5 and (e) 1.25 wt%. (f) Photograph of pure Ni foam, GPN on Ni foam and MoS2/GPN with different (NH4)2MoS4 content on Ni foams. SEM-EDS mapping images of MoS2 (0.5 wt% (NH4)2MoS4)/GPN on Ni foam : (g) BSE image and individual elemental mapping images with different colors of (h) C-green, (i) Mo-purple, and (j) S-dark pink. 23



Figure 2. Raman spectroscopy recorded at an excitation wavelength of 532 nm (a) GPN on Ni foam, MoS2 ((b) 0.2, (c) 0.5 and (d) 1.25 wt% (NH4)2MoS4)/GPN on Ni foam, (e) GPN/Ecoflex and MoS2 ((f) 0.2, (g) 0.5 and (h) 1.25 wt% (NH4)2MoS4)/GPN/Ecoflex.

24


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Figure 3. (a) Photograph of the tapping machine used for pressure measurement. (b) Plot of relative resistance response vs. applied pressure curves for the GPN/Ecoflex and MoS2 (0.2, 0.5 and 1.25 wt% (NH4)2MoS4))/GPN/Ecoflex pressure sensors, respectively. (c) Sensitivity under three linear slope regions for the GPN/Ecoflex device, and MoS2/GPN/Ecoflex devices with different (NH4)2MoS4 content, respectively. (d) Time dependence of the relative variation in resistance for the MoS2 (1.25 wt% (NH4)2MoS4))/GPN/Ecoflex pressure sensor under different pressure. (e) Real-time monitoring of the relative resistance variation in device

during

1,000

seconds.

(f)

Durability

test

of

the

MoS2 (1.25

wt%

(NH4)2MoS4))/GPN/Ecoflex pressure sensor under repeated pressure cycles from 1 to 4,000 with an applied pressure of 5.08 kPa.

25



Figure 4. (a) Photograph of the tensile testing machine. (b) Relative resistance variation versus time curve of the MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor as the increase of tensile strain. (c) Relative change in resistance versus bending strain curve of MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex sensor. (d) Time dependence of the relative resistance 26


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change of the sensor during repeated bending-relaxation with increasing bending strain. The inset shows photo images of the bending machine under different bending strain. (e) Photograph of the the MoS2 (1.25 wt% (NH4)2MoS4)/GPN/Ecoflex with various movements such as (i) bending, (ii) stretching, and (iii) twisting, and (iv) the large scaled Ni foam and the MoS2/GPN grown on Ni foam (10 × 10 cm2). The relative resistance response of the MoS2/GPN/Ecoflex sensors corresponds to various motion signals such as (f) finger bending, (g) eye blinking and (h) forward bending of the neck.

27



Figure 5. (a) Schematic illustration of the flexible 3 × 3 tactile sensor array fabricated by the MoS2/GPN/Ecoflex. (b) Photograph of the 3 × 3 tactile sensor array. Time-dependent relative resistance changes of the 3 × 3 tactile sensor in the array point of (c) a‒3 and (d) b‒1, respectively during the pressure test. (e) Real-time variation in resistance as a function of array point when the electrode connected in the array point of b‒2. (f) Bar graph showing the distribution of relative resistance changes for the 3 × 3 tactile sensor array with the array point of b‒2. 28


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TABLE OF CONTENT

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Highly Sensitive and Flexible StrainâPressure Sensors with Cracked

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