MoS2-decorated laser-induced graphene for a highly sensitive

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Functional Inorganic Materials and Devices

MoS2-decorated laser-induced graphene for a highly sensitive, hysteresis-free and reliable piezoresistive strain sensor Ashok Chhetry, Md. Sharifuzzaman, Hyosang Yoon, Sudeep Sharma, Xing Xuan, and Jae Yeong Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04915 • Publication Date (Web): 05 Jun 2019 Downloaded from http://pubs.acs.org on June 5, 2019

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ACS Applied Materials & Interfaces

Mos2-Decorated Laser-Induced Graphene for a Highly Sensitive, Hysteresis-Free and Reliable Piezoresistive Strain Sensor Ashok Chhetry, Md. Sharifuzzaman, Hyosang Yoon, Sudeep Sharma, Xing Xuan and Jae Yeong Park*

Department of Electronic Engineering, Kwangwoon University, 447-1 Wolgye-dong, Nowongu, Seoul 01897, Republic of Korea

*Corresponding author: [email protected] (Jae Yeong Park)

ABSTRACT: Advancement of sensing systems, soft robotics, and point-of-care testing requires the development of highly efficient, scalable, and cost-effective physical sensors with competitive and attractive features such as high sensitivity, reliability, and preferably reversible sensing behaviors. This study reports a highly sensitive and reliable piezoresistive strain sensor fabricated by one-step carbonization of MoS2-coated polyimide (PI) film to obtain MoS2-decorated laser-induced graphene (MDS-LIG). The resulting three-dimensional porous graphene nanoflakes decorated with MoS2 exhibit stable electrical properties yielding reliable output for longer strain/release cycles. The sensor demonstrates high sensitivity (i. e. gauge factor, GF  1242), is hysteresis-free (~ 2.75%), has a wide working range (up to 37.5%), ultralow detection limit (0.025%), fast relaxation time (~ 0.17 s), and a highly stable and reproducible response over multiple tests cycles (> 12,000) with excellent switching 1 ACS Paragon Plus Environment

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response. Owing to the outstanding performances of the sensor, it is possible to successfully detect various subtle movements ranging from phonation, eye-blinking, and wrist pulse to large human-motion-induced deformations.

KEYWORDS: MoS2-decorated laser-induced graphene (MDS-LIG), piezoresistive strain sensor, laser-irradiation, crack propagation, subtle strain 1. INTRODUCTION Highly sensitive, flexible, and stretchable strain sensors with low hysteresis and a linear strain response are desirable for the detection of subtle human motions detection, such as wristpulse, phonation, and health-care applications1,2. Furthermore, fast responsiveness and stability are other crucial parameters in the accurate monitoring of external stimuli over an extended time. Despite tremendous progress in the field of nanomaterials, straightforward synthesis of such sensors in a scalable approach, enabling high sensitivity and stability over a long period is still challenging for potential applications in wearable electronics, soft robotics, and healthcare monitoring. Various materials such as silver nanowires (AgNWs)3–5, carbon nanotubes (CNTs)6,7, carbon black particles (CB)8, reduced graphene oxide (rGO)9–11, ionic liquids12,13, metal nanoparticles14,15, metal oxides (ZnO and TiO2)16–18, etc. are typically considered as the active materials for the development of flexible strain sensors. Some of the results are impressive; however, the majority of these studies report either low gauge factor (GF) or nonlinear response, resulting in low reliability for repeated strain/release cycles. For instance, Yu et al. developed a highly flexible and transparent strain sensor by stacking superaligned CNT films on a polydimethylsiloxane (PDMS) substrate19. Although the sensor had a strain limit reaching up to 400%, the resistance of the sensing material lacked reversibility upon strain release. Generally, at large-scale strain, change in resistance is the ultimate result of the complete loss of electrical connection with irreversible degradation; subsequently, high 2 ACS Paragon Plus Environment

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values of GF have been demonstrated20,21. For such sensors, high sensitivity and linearity under small-scale strain with a reversible electrical connection for repeated strain/release cycles are the main challenges to fulfill their potential applications. Among two-dimensional (2D) nanomaterials, graphene is the most intensively studied due to it’s excellent mechanical, chemical, and electronic properties10,11,22. The unique physiochemical properties make graphene a prospective candidate material for applications in supercapacitors23, electrochemical biosensors24, and physical sensors10,22,25,26. Graphene can be produced from various carbon sources such as laser exfoliation of highly ordered pyrolytic graphite27 or graphite oxide (GO)23,24 and laser-induced chemical vapor deposition28. Apart from these methods, direct laser-scribing on carbon-rich precursors (PI, wood, etc.) under CO2 laser can also generate porous graphene networks29,30. To date, laser-induced graphene is found to be a robust and rapid method for preparing three-dimensional hierarchical networks of porous graphene on flexible substrates23,29,31. The technique provides a rapid route towards a simple, facile, and scalable approach for patterning variety of carbon precursors that can be converted into amorphous carbon31. Graphene obtained from this technique has stable electrical performance; however, when used in the strain sensing applications, the repeated number of strain/release processes lead to structural damage of the graphene flakes, thereby increasing the resistance of the film irreversibly. Ren and co-workers32 employed a simple lift-off process to remove the unreduced graphene oxide present in LIG. After laser-scribing of graphene oxide, the sheet resistance of the total graphene structure decreased to 700 /sq., improving the GF up to 673. However, the sensor has a nonlinear response with a narrow working range (up to 10%) and poor mechanical durability up to few hundreds of cycles. Recently, Gao et al.33 patterned the features by direct laser-scribing on Ecoflex polymer, thereby converting the material into large band gap silicon carbide (SiC) without the need of any post-synthesis. Although the GF achieved was excellent, the device suffered from overshoots in incipient cycles of mechanical durability test, and also the device has limited 3 ACS Paragon Plus Environment


stretchability (< 5%) due to the fragility of the SiC. Therefore, to meet the demand of emerging wearable applications, highly sensitive, hysteresis-free, and reliable strain sensor with an efficient fabrication technique appears to be essential. 2D transition metal dichalcogenides (TMDs), in particular, molybdenum disulfide (MoS2), has received much attention not only because of its graphene-like structure but also its unique chemical and physical properties34–36. The electronic property of MoS2 can vary from semiconductor (2H-MoS2) to metallic (1T-MoS2), depending on its crystalline phase. MoS2 exhibits a sizeable bandgap varying from 1.2 eV (bulk material) to 1.8 eV (monolayer), thereby becoming a novel candidate material in nanoelectronics for high-performance and low-power device applications37. By taking the advantages of high mobility of graphene and structural rigidity of MoS2, the electromechanical performance of MoS2-decorated laserinduced graphene (MDS-LIG) has been improved due to the interfacial coupling between neighboring layers of van der Waals heterostructures (vdWHs)38,39. The lattice dynamics of vdWHs formed by two different types of 2D materials, such as semimetal (graphene) and semiconductor (MoS2), is expected to engender synergistically coupled nanohybrid having robust mechanical properties with higher surface area34,40. The reasonable composition of MoS2 with highly conductive graphene may limit the electrical conductivity of the MDS-LIG nanohybrid, but the intrinsic active hierarchical structure of MoS2 and synergistic interaction between the composited layers mitigates the fracture of graphene nanosheets up to certain strain limits. Concerning these points, an interfacial interaction between nanohybrid was optimized by modifying the surface morphology of MoS2 in an efficient way without using any binders or additives. CO2 laser-irradiation of multilayered MoS2 is a reliable method for the fabrication of sheet-like few-layer MoS2 with properties comparable to pristine monolayer MoS2. Additionally, monolayers MoS2 exhibit a high Young’s modulus and breaking strength, making them promising strain-dependent materials41. Furthermore, the layered

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structure of MoS2 allows binding of foreign atoms to provide rigidity to the composite material, which is highly advantageous for higher cycling stability of the sensor34. In this paper, we propose a strategy for the fabrication of a highly sensitive and stable strain sensor based on MDS-LIG for the implementation of TMDs in flexible and wearable electronics. The laser-scribing of multilayered MoS2 coated on commercial PI film using a CO2 infrared laser yields three-dimensional porous graphene decorated with MoS2 enabling superior electromechanical properties. During the laser-scribing process, atomically thin films are obtained by photothermal conversion of sp3-carbon atoms into sp2-carbon atoms, thereby leading to porous MDS-LIG heterostructures. The resistance response of the sensor exhibits high sensitivity (GF  1242), low hysteresis, ultralow limit of detection, and a broad working range, along with long-term reliability (> 12,000 cycles). Finally, the feasibility of the sensor is confirmed with large human body movements to subtle deformations within the human body such as phonation and wrist pulse.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MoS2. A facile hydrothermal method was adopted for the preparation of MoS2 nanosheets42. All chemical reagents were of analytical grade and used without further treatment. A quantity of 1.1 g of sodium molybdate dihydrate (Na2MoO4.2H2O) and 1.0 g of thiourea (NH2CSNH2) was added to 35 mL deionized (DI) water and magnetic stirred for about 10 min. The pH value of the composite was adjusted to less than one by adding 12 M HCl. Then, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 24 h. The schematic for the synthesis of MoS2 is illustrated in Figure S1, Supporting Information. Finally, after natural cooling, the black MoS2 was collected by vacuum filtration and washed several times with copious amounts of DI water, and then dried at 60 °C for 24 h. The powder of MoS2 (4 mg) was exfoliated by vigorous ultra-sonication for 5 ACS Paragon Plus Environment


1.30 h at 10 °C in a 10 mL mixer solution of n-methyl-2-pyrrolidone (NMP) and DI water (1: 1 vol. ratio) to produce the homogeneous dispersion. 2.2. Fabrication of MDS-LIG sensor. A three-dimensional porous network of MDS-LIG was prepared by CO2 infrared laser-scribing (Microlaser C40, Coryart, Korea) of MoS2coated commercial Kapton PI sheet (200 m). Laser-scribing was performed using a 10.6 m CO2 laser at fixed dots per inch (DPI) of 1,000 at various power, gap, and speed. The optimized laser parameters with a power of 7.2 W, a gap of 0.075 m, and speed of 150 mms1 (linear laser density of ~ 51 Jm-1) at ambient conditions were adopted for the fabrication of the final device. To coat a thin film of MoS2 on the polymeric substrate, asprepared MoS2 solution (100 L) dropped cast on a tape-fixed area of 48 × 4 mm, and the solvent was evaporated at 120 °C for 30 min. The PDMS prepolymer and its cross-linking agent (Sylgard ® 184, Sigma-Aldrich) was prepared in 10: 1 weight ratio, degassed and then coated by doctor blading method (thickness of ~ 0.5 mm). After PDMS curing at 85 C for 2 h, the exterior of MDS-LIG was peeled off from the PI substrate. The electrical contacts were made by connecting the copper tape with the help of silver paste. To obtain the final sensing unit, the transferred patterns were again encapsulated by the PDMS suspension and then cured, followed by cutting and electrode exposure. 2.3. Characterizations. The surface morphology of MDS-LIG was investigated by highresolution FESEM (JSM-6700F, Field Emission Scanning Electron Microscope) images. For the elemental analysis, Energy-dispersive X-ray spectroscopy (EDS) was also performed using the same instrument. Raman shifts were obtained by Renishaw (inVia Raman Microscope) using a 514-nm excitation laser. X-ray photoelectron spectroscopy (XPS) spectra were measured using a PHI 5000 VersaProbe (ULVAC PHI, Japan). Fourier-transform infrared spectroscopy (FTIR) spectra were recorded in the range 5004000 cm1, employing


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Thermo scientific NICOLETiS10. All of the samples for analytical characterizations were the MDS-LIG films on the PI sheet. A moving fixture (JSV-H1000, Japan Instrumentation System Co., Ltd.) and force gauge (HF1, Japan Instrumentation System Co., Ltd.) in combination with homemade clampers were used to apply longitudinal tensile strain. During the electromechanical characterizations, the cross-sectional area of the sensor in which tensile force applied was 1.29 mm  17.62 mm. Instantaneous resistance measurements were performed using an LCR meter (Hioki, IM 3536) at a dc voltage of 0.5 V. The surface resistance of the LIG porous graphene was measured by a four-point meter (RC3175, EDTM). Optical images were taken by AcquCAM Pro/U microscope with Olympus, UMPlan FI 10x 0.25 BD lens.

3. RESULTS AND DISCUSSION 3.1. Fabrication of MDS-LIG. The flexible and stretchable strain sensor consists of hierarchical networks of porous MDS-LIG produced by direct laser-scribing of MoS2-coated PI film (for details, refer to the Experimental Section) encapsulated within a PDMS elastomer. As depicted in the schematic of Figure 1a, MoS2-coated PI was placed in the focused laser beam of a wavelength of 10.6 µm under ambient conditions. Studies reveal that the fabrication of large-scale porous graphene is possible without the need for an inert atmosphere due to oxidation at the laser scribed zone. The morphological difference in the carbonized patterns can be observed depending on the scan line gap as depicted by FESEM images of Figure 1b (0.1 mm) and Figure 1c (0.075 mm). The ridges are formed in an orderly fashion along the carbonized direction. As the line width is ~ 75 m, similar to the spot diameter of the laser, carbonization in the targeted area requires multiple sweeps of laser beams. At low power densities, the crystalline graphene disperses into the nanocrystalline structure, and for power densities higher than the threshold, hydrogenated amorphous carbon atoms liberate on the surface43. The high-magnification FESEM image of Figure 1d and its inset clearly shows 7 ACS Paragon Plus Environment


that CO2 laser scribing of MoS2-coated PI film resulted in a hierarchical porous network of MDS-LIG (Figure S2a, Supporting Information). As seen from the FESEM images, the nanostructure is composed of a wrinkled flake-like morphology, which results from the rapid liberation of carbonaceous and gaseous products43. A cross-sectional view of the MDS-LIG (thickness of ~ 47 µm) and PI substrate is depicted in FESEM image of Figure S2b, Supporting Information. Graphene formation is most likely a photothermal process, in which high localized temperatures of > 2,500 ºC can easily break the CO, C=O, and CN bonds of the PI polymer29. When PDMS suspension cast on the MDS-LIG; PDMS infiltrates into the interconnected channels of a hierarchical porous network of MDS-LIG due to the low surface energy and low viscosity of the PDMS. After curing of the PDMS elastomer, the MDS-LIG pattern was peeled off from the PI substrate. Figure 1e shows the photographs of i) the replication of MDS-LIG into PDMS, ii) twisting, and iii) the final sensor. The MDS-LIG patterns can be easily folded and twisted by any desired angles (Figure S2c, Supporting Information). The schematic of Figure 1f illustrates the chemical structure of MoS2 depicting the sandwiched layers. As MoS2 exhibits a hexagonal structure with each monolayer, three stacked layers of SMoS are covalently bonded by weak intermolecular forces. Their common characteristics are strong intra-layer covalent bonding among layered structures and weak van der Waals forces between the interlayer sheets34,36. The intralayer transition metalchalcogen bonds are predominantly covalent in nature, whereas sandwiched layers are coupled by weak van der Waals forces, such that charge transport occurs between the nanosheets38.


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Figure 1. Fabrication process and morphology of MDS-LIG strain sensor. (a) Schematic illustration of the MDS-LIG strain sensor fabrication. The low-power infrared laser irradiation of MoS2-coated PI directly converts the film into well-exfoliated 3D-hierarchical networks of porous graphene decorated with MoS2. (bc) FESEM images showing the ridge scan line patterns at 0.1 mm and 0.075 mm, respectively, at the laser power of 7.2 W and raster speed of 150 mms1. (d) High-resolution FESEM images of hierarchical porous graphene decorated with laser-irradiated few-layer MoS2 nanosheets. (e) Photograph showing i) the successful transfer of MDS-LIG into PDMS substrate, ii) twisting, and iii) the final sensor. The structure is not only stretchable but also mechanically stable which can be twisted and bent without any loss of electrical performance. (f) Chemical structure of two layers MoS2 showing the single layer of molybdenum is sandwiched between two layers of sulfur.

In a typical experiment, the influence of laser-irradiation on multilayered MoS2 topography was studied by means of FESEM. The FESEM image of Figure 2a shows the multilayered MoS2 deposited onto SiO2/Si substrate before laser-irradiation. After CO2 laser-irradiation of multilayered MoS2, the topography resulted in a large-area, sheet-like MoS2 flakes as shown in FESEM image of Figure 2b. It was expected that the change in crystallinity occurred due to 9 ACS Paragon Plus Environment


adsorption of CO2 from an infrared laser resulting in sheet-like morphology36. In addition, the localized high temperature provided the unique reaction root for the effective dispersion of MoS2, thereby enhancing the surface area for electron transport40,44. Moreover, the influence of laser parameters (power, gap, and scanning speed) was also studied for the optimization of electrical conductivity of carbonized patterns, and to ensure successful replication of the pattern into the elastomeric substrate. Over-burning of the PI film results in the blow-off of graphene flakes. Similarly, complete transfer of the carbonized pattern becomes impossible when the polymer does not sufficiently burn or turns into featureless. It is noted that low thermal energy (< 6 W) and slow scanning speed (< 100 mms1) do not sufficiently carbonize the polymer, which ultimately hinders the successful replication process. As shown in Figure 2c, the graphene flakes obtained at 7.2 W and 120 mms1 offer the lowest sheet resistance of 57.6 /sq., but the problem of overcutting through the substrate was observed. The conductivity of the scribed pattern is also dependent on the laser scan gap (Figure 2d). A scanning gap larger than spot diameter leaves an unburned area in between the two scan lines. On the contrary, a smaller scanning gap overlaps the carbonized area, causing overburning of the polymer. Likewise, the high speed of the laser tip roughly burns the polymer, causing less graphitization (Figure 2e). Considering these parameters, we adopted the scribing power, line gap, and scanning speed of 7.2 W, 0.075 mm, and 150 mms1, respectively, for the fabrication of the final MDS-LIG strain sensor. The resulting MDS-LIG sensor exhibited the resistance of 175.4 , comparable to strain sensors using CNT (~ 100 ) and AgNWs (~ 250)3. As shown in Figure 2f, the general test for the conductivity of the MDS-LIG with a gradual increase in the strain was performed by connecting with LED (2 V/20 mA) to 5V DC power supply. The resistance increases with the tensile strain (0%15%), as indicated by diminishing of LED intensity.


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Figure 2. Performance characteristics of laser-irradiated MoS2 and PI. FESEM images of multilayered MoS2 onto SiO2/Si substrate (a) before laser-irradiation, and (b) after laser-irradiation. The laserirradiation of multilayered MoS2 down to few-layer MoS2 provides a reliable method to fabricate large-area nanosheets having electrical conductivity comparable to 1T-MoS2. The linear resistance of laser-induced graphene as a function of (c) scribing power, (d) line gap, and (e) raster speed. The scribing power, line gap, and raster speed of 7.2 W, 0.075 mm, and 150 mms1, respectively were adopted for the fabrication of the final device. (f) Photographs showing the diminished intensity of LED as a function of tensile strain: i) 0%, ii) 2.5%, iii) 7.5%, and iv) 15%.

3.2. Analytical Characterization. The crystallinity, elemental analysis, chemical interactions, and compositions of MDS-LIG were systematically studied by Raman spectroscopy, XPS, FTIR, and EDS, respectively. In the Raman spectrum of Figure 3a, three prominent peaks, namely the D peak (induced by defects or bent sp2 bonds) at ~ 1,349 cm1, G peak (ordered sp2 structures) at ~ 1,579 cm1, and 2D peak (related to second-order zoneboundary phonons) at ~ 2,687 cm1 were observed. MoS2-related peaks A1g at ~ 380 cm1 (due to opposing vibration) and E2g at ~ 408 cm1 (related with the in-plane vibration of S atoms with Mo atoms) were also observed at low wave numbers35. The peak at ~ 2936 cm1 indicates the multilayer structure of the graphene (D+G band)21. The ratio of peak intensity (ID/IG = 0.81) shows a reasonable number of defects in MDS-LIG (Figure S3a, Supporting Information). The chemical composition of the active materials contained in our sensing 11 ACS Paragon Plus Environment


material was further confirmed by high-resolution XPS spectra. The survey spectra acquired on MDS-LIG (Figure S3b, Supporting Information) sample confirms the presence of C, O, Mo, and S elements, which was also confirmed by the EDS spectra (Figure S4, Supporting Information). The atomic concentration ratio of C/O = 6.52 and absence of chemical element N provided initial confirmation of the successful conversion of the PI into few-layer graphene.

Figure 3. Analytical characterization of the MDS-LIG heterostructure. (a) Raman spectra with and without MoS2 showing the presence of D, G, and 2D peaks. The ratio of peak intensity (ID/IG = 0.81) shows a reasonable number of defects in MDS-LIG. (b) XPS spectra of MDS-LIG composite showing the different deconvoluted peaks. The C1s spectrum of XPS indicates the dominant portion of the CC peak (sp2-carbon atoms) of LIG in the composite. (c) High-resolution XPS spectra of Mo 3d. (d) FTIR spectra comparison between PI, MoS2, and MDS-LIG. The MDS-LIG spectrum shows the absorption of the peaks of PI after laser-irradiation.

Furthermore, high-resolution acquisition of the most significant C1s region of the XPS spectra (Figure 3b) shows that the CC peak and shake-up peak are centered at 284.54 eV and 291.04 eV, respectively, which is distinctive of graphitic structure35. The spectra of the Mo 3d region 12 ACS Paragon Plus Environment

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in Figure 3c confirms the presence of MoS2 flakes. The intense peaks at 228.54 eV and 231.67 eV attributed to the presence of Mo 3d5/2 and Mo 3d3/2 states, respectively, and the S2s peak at 226.42 eV, all of which depict features of MoS22. The FTIR spectra of Figure 3d illustrate the broad absorption of distinct peaks at 5141775 cm1 in PI (corresponding to stretching and bending modes of CO, C=O, and CN bonds) after laser-scribing leading to large variations in the local environment. 3.4. Electromechanical Performance. The current-voltage (I-V) characteristic of an MDSLIG strain sensor for various strains (0%37.5%) is depicted in Figure 4a. The current increases monotonically with bias potential, obeying the linear relationship of Ohm’s law. Meanwhile, it is observed that the slope of the I-V curve decreases with increasing strains owing to the relative increase in resistance, which confirms the piezoresistive nature of the MDS-LIG strain sensor. For strains higher than 37.5%, the I-V curve for the MDS-LIG strain sensor is nearly flat, implying infinite resistance between the conductive pathways. As shown in Figure 4b, the relative resistance changes recorded for strain releasing follows the same path as traced during stretching, indicating negligible hysteresis (~ 2.75%) of the sensor (see Figure S5, Supporting Information for all ranges of strain). The average value of the hysteresis (h) was calculated from the resistance (R) values as follows, h(%) 

Rs  Rr 100% Rm

(1)

where the subscripts s, r, and m represent the stretching, releasing and the maximum value of the resistance at a particular strain, respectively. When MDS-LIG is subjected to strain, the interspaces between graphene flakes linearly increase with increasing strain due to the deformation of the PDMS substrate. For the strain exceeding 37.5%, there is a remarkable change in resistance due to the structural damage of the graphene flakes causing the irregular response. Within strain limit of 37.5%, change in resistance response is linear and reversible 13 ACS Paragon Plus Environment


because the slippage mechanism within the graphene flakes is dominant over the crack formation mechanism. Due to the incorporation of MoS2, minimal cracks are formed and reconstructed back with a little effect on the resistance response at lower strains. The exceptionally high strength and defect-free property of MoS237 plays a significant role in the restriction on excess breakage of the graphene flakes. As such, the sensor offers a linear and reversible response attributed to the homogeneous crack formation in the structure45. Therefore, we restrained the working strain limit below 37.5% during all other electromechanical characterization since the linear regime is of particular interest for precise measurement of subtle strains.

Figure 4. Electromechanical characterization of the MDS-LIG strain sensor. (a) The linear relationship of I-V curves for verification of Ohm’s law. The slope of the curve decreases for the strain from 0%37.5% (stretching by 15 mm) showing increasing nature of the resistance. (b) Relative resistance change vs. strain during stretching (green line) and releasing (red line) up to a safer limit of 25%. The negligible value of hysteresis of ~ 2.75% indicates that change in resistance behavior is fully reversible. (c) Relative change in resistance vs. strain comparison between MDS-LIG and LIG strain sensor for the measurement of the sensitivity (i. e. gauge factor, GF). The sensor offers the GF of 236.2 at < 16.7% strain and increases to 1242 beyond 16.7%. (d) Statistical comparison of GFs of


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MDS-LIG strain sensor with that of LIG only. (e) The comparison of the dynamic response of the MDS-LIG sensor at a different strain of 5% (red line), 12.5% (orange line), and 20% (green line) strain. For each strain, the sensor was stretched for 5 s and released for another 5 s. (f) A representative plot illustrating the response and relaxation time. Due to the incorporation of MoS2, the stiffer material, the sensor regains its original state more rapidly as implied by faster relaxation time.

The common methods for the fabrication of the strain sensor are either the deposition of thin conductive nanomaterials7,10 or embedding of conductive polymers46,47/ionic liquids12 into the stretchable substrate. In such sensors, the limit of stretchability merely depends on the nature of the substrate material and hence, change in the resistance is influenced by the modulus of the substrate7,19,48. At high strains, the large mathematical value of GF calculated from extremely high resistance change is an ultimate result of the complete loss of electrical connection33,49. Although such sensors offer high GFs, nonlinearity and overshoot in the resistance response limits their practical application in subtle human skin deformations. In spite of high GF, the linearity among the resistance values best determines how accurate the sensitivity is throughout the applied strains50. Therefore, the GF with linearity is the preliminary metric to precisely measure the performance of such sensors. To compare the performance of our sensor with other reports, we calculated the GF using the expression, GF 

  R  Ro  Ro 

(2)

where Ro and R are the initial resistance and resistance at a particular strain  ( L L) , respectively. Under tensile strain, the relative resistance change  R  Ro  Ro increases monotonically with increasing strain. The strain sensitivity is obtained from the slope of the trace of relative resistance change versus tensile strain, as depicted in Figure 4c. The MDSLIG sensor offers a GF of 236.2 up to a strain of 16.7% and then increases to 1242 beyond 16.7%. For the performance comparison, a sensor with LIG only was also fabricated following the method same as used for MDS-LIG. Figure 4d reveals that the LIG sensor decorated with MoS2 offers the higher GF than that of LIG only counterpart. To the best of 15 ACS Paragon Plus Environment


our knowledge, the GF of the MDS-LIG-based strain sensor is higher than those of the recently reported piezoresistive strain sensors10,32,51–54. We also investigated the GF of the sensor subjected to the compressive strain along the longitudinal direction. For a compressive strain of 77.5% (stretched by 31 mm), the GF of the sensor was found to be ~ 3.67 with a negligible residual  R  Ro  Ro of 0.16 (Figure S6, Supporting Information). Along with a high strain sensitivity, a low value of initial resistance is also desirable for the low-power electronics applications. Our sensor offers a low initial resistance of 175.4 , indicating its usefulness for low-power applications. Furthermore, the dynamic response of the sensor was assessed by subjecting various strains ranging from 5%20%. In each strain, the sensor was stretched and hold for 5 s followed by a release for another 5 s while continuously measuring the resistance (Figure 4e). The response of the sensor consistently maintained for the period of strain applied and regained to its original value upon release. A plot illustrating the sensor’s dynamic response and relaxation time is shown in Figure 4f. The dynamic response (and relaxation) time depends on how fast strain responds to strain loading (and unloading). With the maximum possible stretching speed of 600 mms1, the time interval between adjacent levels shift was recorded as 0.25 s (during stretching) and 0.17 s (during releasing) indicating faster recovery upon release. The relaxation time is significantly improved due to the incorporation of stiffer material, MoS2, which worked as a bridge to bind graphene flakes to recover electrical connection upon strain release.


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Figure 5. The response of the MDS-LIG sensor to various strains. (a) Schematic illustration of the sensor under no strain and tensile strain conditions. (b) Optical images showing the formation of cracks at various applied strains: i) 0%, ii) 20%, iii) 37.5%, and iv) 87.5%. The cracks are uniformly formed, and reversible up to the strain of 37.5% providing a linear resistance response. The direction of the double-headed arrow and its length represents the direction of strain and magnitude, respectively. (c) Relative change in resistance response for a cyclic strain of 0%0.025%0% for the determination of the lower limit of detection. (d) Relative change in resistance response at low strains of 2% and 5%. (e) The time-dependent resistance response of the MDS-LIG strain sensor for nine consecutive stretch/release cycles of tensile strains of 2%, 5%, 10%, 15%, and 20%. (f) Staircase response to the sequential stretch and release of the sensor by the step size of 2 mm. In every step, the sensor was stretched/relaxed by 2 mm from the previous state and hold for 5 s to analyze the response. (g) Relative change in the resistance for relatively higher tensile strains of 10% and 20%. (h) Electromechanical cyclability of the sensor for the strain of 4% enduring stability over 12,000 cycles. (i) Similar representative analysis at the strain of 8% for 200 cycles. The changing nature of resistance is stable and reversible under both small and large strains, indicating high stability of the sensor.

Figure 5a depicts the schematic of the sensor before and after straining. During loading of strain, the materials undergo residual stresses55–57 originated between MDS-LIG and PDMS 17 ACS Paragon Plus Environment


substrate, causing the formation of cracks. An optical microscopy images of Figure 5b depicts the mechanism of crack propagation during loading of the strain. The cracks are formed in the direction perpendicular to the strain loading as represented by double-headed arrows. For a gradual increase in strain up to 37.5%, the noticeable cracks were observed still preserving the structure of MDS-LIG. At this stage, the cracks have negligible influence in the resistance overshoot yielding better linearity. However, for the strains larger than 37.5%, the enlarged cracks play the dominant effect in the electrical connection leading to a dramatic change in resistance. As a result, high GF is expected; however, the device loses linearity and reversibility in resistance response due to structural damage of the MDS-LIG. Due to the merit of high sensitivity, the lower limit of detection was examined by applying the incremental strain while continuously observing the response. As shown in Figure 5c, the sensor can detect cyclic strain of 0%0.025%0% (stretched by 0.01 mm), lower than the best-reported values32,33,54,58. The performance of the sensor was then subsequently analyzed for increasing values of the strain of 2% (black line) and 5% (red line) as shown in Figure 5d. The results demonstrate that the relative change in resistance response is excellent under each strain conditions and recovers completely upon release. A similar representation from small to large strains (2%20%) as a function of time over nine cyclic stretch/release is illustrated in Figure 5e. The equivalent cyclic response is depicted in Figure S7, Supporting Information. Under large strains, the junction resistance between graphene flakes may increase significantly; however, MoS2 nanosheets can serve as additional pathways for conduction between graphene flakes, thereby successfully maintaining the electrical conductivity. As seen from Figure 5e, the time required for nine cycles of stretch/release is different for 2% strain (~ 10 s) and 20% (~ 23 s). This time variation to different strain input is attributed to the fact that the stretch/release process is a quasi-transient process (Figure S8, Supporing Information) requiring more time for large elongations as discussed before. Similarly, the staircase response of the sensor was investigated by sequentially stretching the sensor by 2 18 ACS Paragon Plus Environment

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mm (5% strain) and holding for 5 s, as shown in Figure 5f. When the strain reached 20%, it was released with the same step size as applied during the stretching. The responses to stretching and releasing profiles were symmetrical to each other, indicating linear and reversible nature of the sensor (Figure 5f). Figure 5g presents the cyclic response of the sensor for relatively higher strains of 10% (blue line) and 20% (orange line). As seen from the figure, the sensor preserves its initial resistance in each successive cycle due to the intrinsic properties of heterostructures, in which cracks are re-bridged during the release of the applied strain59,60. The dominant slippage mechanism along with crack propagation of graphene flakes and the crack repairing ability of MoS2 is responsible for the high GF and reversible electrical connection, respectively. Furthermore, to evaluate the long-term durability of the sensor, the electromechanical stability of the sensor was carried out at 4% strain for over 12,000 strain/release cycles. During the cyclic test, there was no significant difference in the sensor response (Figure 5h). The device can perform well after 12,000 cycles, efficiently maintaining its electrical performance. We noticed that the value of  R  Ro  Ro was decreased from an initial value of 3.33 to 3.04 even after 12,000 cycles of electromechanical tests indicating long-term durability of the sensor. Meanwhile, a representative test performed for a relatively higher cyclic strain of 8% for 200 cycles is illustrated in Figure 5i. As seen from the figure, the deviation noticed is slightly higher (relative change in resistance from 8.54 to 7.71) than that of cyclic response for 4% strain. In brief, the sensor’s response was consistent without any severe degradation in its performance. The high durability of the sensor is ascribed to reversible nature of crackrepairing upon strain release. The reproducibility of the sensor was confirmed by testing the response of three identical samples subjected to the strain of 5% for nine strain/release cycles (Figure S9, Supporting Information). The performance of our sensor along with recently



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reported piezoresistive strain sensors are compared in Table 1 (Figure S10, Supporting Information). Our sensor exhibited significantly improved GF with stable and reliable results. Table 1. Performance comparison of the state-of-art piezoresistive strain sensor GF/strain range (%) lower limit

upper limit

response time

stability

ref

CB and cellulose on paper

4.3 (00.6)



240 ms

1,000

8

Graphene/ecoflex

0.6 (< 5)

17.4 (5 <  < 200)

0.11 s



10

Super-aligned CNT

0.12 (0100)

0.2 (300400)

98 ms

5,000

19

LSGa

mesh/ecoflex

58 (02)

673 (4.55)



1,000

32

TPEb-wrapped SWCNT

48 (05)

425 (20100)



3,250

50

MWCNT-PDMS

55.8 (< 70)



90 ms

10,000

51

CNT gradient structure

7.7 (0200)

13.5 (200500)

33 ms

12,000

52

AgNWs on dragon skin

24.6 (0130)

81 (130150)



10,000

53

Ti3C2Tx MXene/CNT

64.6 (030)

772.6 (4070)



5,000

54

AuNWs/latex

9.9 (05)

6.9 (550)

< 22 ms

> 5,000

57

Fish-scale-like rGO layers

16.2 (< 60)

150 (> 60)



> 5,000

58

Carbonized cotton fabric

25 (080)

64 (80140)



2,000

59

Conductive carbon thread

8.7 (04)

18.5 (810)



2,000

61

Carbonized melamine

5 (040)

18.7 (> 40)

0.24 s

10,000

62

rGO decorated PU mat

11 (010)

79 (< 100)

200 ms

6,000

63

MDS-LIG

236.2 (016.7)

1242 (< 25)

0.25 s

12,000

this work

sensing material

aLSG

= laser-scribed graphene

bTPE

= thermoplastic elastomer

3.5. Application. Due to the excellent performance of the sensor, subtle deformations of the skin such as the wrist pulse and phonation, and movement of the fingers were successfully detected. For vigorous human-motion-induced deformation, the sensor was fastened on the forefinger joint by tightly wrapping an adhesive bandage (Figure 6a). The change in relative resistance response to bending/relaxing was monitored at different bending angles (Table S1, Supporting Information). The response to a few representative bending angles is shown in Figure 6b. The resistance of the sensor increased upon bending and recovered to its original value when relaxed. Generally, the bending strain of such sensors is calculated from bending radius (or curvature radius)2,15. Here, we deduced an empirical formula to calculate the bending angle back from the relative change in resistance response (Figure S11 and Equations 20 ACS Paragon Plus Environment

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S1S3, Supporting Information). The method adopted here is to circumvent the dependency of the sensor’s response with the bending machine or a particular subject involved in the experiment. The technique might be useful in robotic arms, human-machine interfaces, and sign language translation.



Figure 6. Application of the sensor for various real-time signals. (a) Photographs illustrating the sensor fastened on the forefinger and its bending by various angles. (b) The relative change in resistance response for the sequential bending/relaxing (~ 1 Hz) at various bending angles. (c) Photograph of the sensor attached for the detection of human eye blinking, and (d) its response. (eg) Response of the sensor to the pronunciation of various syllable words. For the recognition of syllable words, the sensor was fixed just above the larynx of an adult. (h) Photograph showing the sensor attached on the wrist. (i) Real-time resistance response of the sensor for wrist pulse. (j) Representative pulse signal of the time-averaged eight pulse periods containing the percussion wave (P-wave) and diastolic wave (D-wave).

Similarly, to detect minute human skin deformations such as eye blinking, the sensor was attached nearer to the eye (Figure 6c). The eye blinking movement was distinctively monitored during the opening and closing of an eyelid. The relative change in resistance response for the instantaneous eye blinking and relaxing for 30 s is shown in Figure 6d. Strain-induced during eye closing/opening is suitable for applications in human-machine interface. To investigate the response of the epidermis movement due to the vibration of the vocal cord, the sensor was attached above the larynx. A volunteer was asked to pronounce different syllable words such as “hello”, “congratulations”, and “Kwangwoon”. Notably, for the cyclic pronunciation of each word, the sensor exhibits identical responses in each cycle while the patterns were different from each other depending on the syllables (Figure 6eg). The repeatability of the detection was excellent in each successive testing. The recognition of fast and delicate phonation implies that the sensor can identify a number of syllables in simple words through the monitoring of electrical signals suggesting potential scope in sound monitoring and recognition. The detection of phonation is important in speech recognition, and in the realization of the conversion of meaningless sound to predesigned sound since many mute people cannot communicate with language, but have the ability to produce certain sounds such as a cough, hum, or scream25,58. To demonstrate the capability of the sensor for subtle-strain detection, we applied the sensor for the human wrist pulse measurement. The sensor was mounted above the radial artery of an adult person by wrapping adhesive bandage as shown in Figure 6h. The corresponding 22 ACS Paragon Plus Environment

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resistance response recorded for 15 s is presented in Figure 6i. The arterial pulse rate, calculated from two successive systolic peaks, was found to be 68 beats per minute with a time interval of ~ 0.84 s. The enlarged view of one pulse period obtained from the timeaveraged signal of eight periods is depicted in Figure 6j. The presence of the percussion wave (P-wave) and diastolic wave (D-wave) was clearly observed. These are the important clinical indicators for the diagnose of cardiovascular disease and arterial stiffness. 4. CONCLUSION In summary, we presented a highly sensitive, reliable, and hysteresis-free strain sensor based on MDS-LIG, suitable as a multiscale sensing platform. The high strain sensitivity with a reversible nature of the sensor is primarily appropriate for the application of subtle deformation of human skin such as arterial pulse, eye blinking, and phonation. This work reports the exploration of solution-processable TMD heterostructures in combination with 2D materials required for low-power and reliable physical sensing platforms. The integration of MoS2 with a porous graphene structure produces synergistically coupled hybrid nanomaterial possessing electromechanical stability, beneficial for longer cycling tests. As a result, the sensor provides a high GF of  1242, negligible hysteresis of ~ 2.75%, low detection limit of 0.025%, wide working range up to 37.5%, and stability over > 12,000 cycles. An excellent linear response of the device, larger GF, and fast response time demonstrates the usefulness of the sensor for subtle deformation within the human body. Moreover, the approach adopted here for the fabrication of MDS-LIG sensor is inherently efficient, scalable, and cost-effective for the fabrication of large-area devices. Considering the outstanding sensing performances, we believe that MDS-LIG-based sensor has potential applications in e-skin, human-motion monitoring, wearable smart electronics, and healthcare applications.



ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: Synthesis of MoS2 nanoparticles, FESEM images of LIG and photographs of sensor’s flexibility, Raman and XPS spectra, atomic concentration of elements in MDS-LIG, hysteresis curves at various strains, response of the sensor for compressive strain, response of the sensor for cyclic stretch/release, illustration of quasi-transient process, reproducibility of the sensor, gauge factor (GF) comparison, and relative change in resistance for finger bending (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Ashok Chhetry: 0000-0001-6933-8972 Jae Yeong Park: 0000-0002-2056-5151 Author Contributions A.C. conceived the idea, fabricated and characterized the device, prepared the figures, and wrote the manuscript. M.S. assisted for the fabrication of MoS2 nanoparticles, Y.H. and S.S assisted with the device characterization and measurements, X.X. helped for the analytical characterizations, and J.Y.P. provided regular guidance to the research and revised the manuscript. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This research was supported by the Technology Innovation Program (20000773, Development of nanomultisensors based on wearable patch for nonhaematological monitoring of metabolic syndrome) and (10053023, The Development of RF MEMS Devices Core Technology for Multi-band IoT System Applications) funded By the Ministry of Trade, Industry & Energy (MI, Korea) and the Bio & Medical Technology Development Program of the NRF grant funded by the Korean government (MSIT) (NRF-2017M3A9F1031270). The authors are grateful to the Micro/Nano Devices & Packaging (MiNDaP) Lab. Members of the Kwangwoon University for their technical discussion and support. REFERENCES (1)

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