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MoS2-Based Tactile Sensor for Electronic Skin Applications Minhoon Park, Yong Ju Park, Xiang Chen, Yon-Kyu Park, Min-Seok Kim,* and Jong-Hyun Ahn* Tactile sensors, in the form of conformal and embedded devices, have attracted intense research interest because of their diverse applications, from electronic skin (E-skin) for robotics to health care monitoring systems. Much effort has been made to develop large-area and high-performance tactile sensors with good sensitivity and mechanical flexibility. To address those issues, various strain sensing materials have been studied. Examples include nanomaterials, such as graphene and carbon nanotubes, and hybrid composites, such as metal particles-polymers and conductive materials-nanostructure polymers.[1–11] Despite high sensitivity with respect to external strain, existing sensors have several drawbacks: high hysteresis, nonlinearity, and poor repeatability.[12,13] In contrast, semiconductor material-based piezoresistive sensors give high sensitivity and stable operation.[14–16] As an alternative, MoS2 semiconductors have recently attracted attention because of their outstanding mechanical and optical transmittance, high gauge factor (GF), and tunable band gap.[17–23] However, no one has yet reported on multicell devices for practical applications that transcend the fabrication of a single cell. Here, we present an ultrathin conformal, MoS2-based tactile sensing array covering an area of 2.2 cm × 2.2 cm. We integrated the sensor with a graphene electrode and interconnect to achieve good mechanical flexibility and optical transmittance in the visible color range. This sensor shows high sensitivity, good uniformity, and linearity even after 10,000 loading cycles. In addition, it provides excellent mechanical flexibility over a strain of 1.98% and good optical transparency over 80%. Our ultrathin tactile sensor fabricated on an ultrathin, transparent plastic substrate presented stable performance even on unusual substrates such as leather and a human fingertip. Figure 1a presents a schematic illustration of a 2 in. quartz tube furnace with three temperature zones for the synthesis of wafer-scale, thin-layer MoS2. We placed 0.5–1.5 g of sulfur (S) and 20–70 mg of molybdenum oxide (MoO3) powders (99.5%, Sigma-Aldrich) in two small quartz tubes separately at the upstream end and 4 cm × 3 cm of SiO2 (300 nm)/Si substrate
M. Park, Y. J. Park, Dr. X. Chen, Prof. J.-H. Ahn School of Electrical and Electronic Engineering Yonsei University 50 Yonsei-ro Seodaemun-gu, Seoul 03722, Republic of Korea E-mail: [email protected] M. Park, Dr. Y.-K. Park, Dr. M.-S. Kim Center for Mass and Related Quantities Korea Research Institute of Standards and Science 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea E-mail: [email protected]
DOI: 10.1002/adma.201505124
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at the downstream end. The setting temperatures for zone I (S powder), zone II (MoO3 powder), and zone III (SiO2/Si substrate) were 110–150 °C, 580– 650 °C, and 650–750 °C, respectively. During the reaction process, we kept the chamber at a low pressure (0.7 Torr) in an Ar atmosphere (flow rate: 100 sccm). After 30–60 min, we turned the furnace off, moved it away, and left the sample in the chamber to cool slowly to room temperature. Compared with the bare SiO2/Si substrate, the color of the as-grown MoS2 sample obviously changed from purple to blue (inset of Figure 1a). The optical microscopic image of the MoS2 layer on SiO2/Si substrate reveals a uniform and continuous color distribution across the wafer (Figure 1b). To estimate the thickness of the synthesized MoS2 layer, we used atomic force microscopy (AFM, XE-100, Park system) to analyze the topography of the area near the border (Figure 1c). The inset is the cross-sectional height profile for the trace, indicated as a red line. The result shows that the thickness of the MoS2 layer is ≈1.4 nm, indicating a bilayer.[24] We used this bilayer MoS2 because it showed the most reasonable characteristics in terms of electrical and optical properties. To measure the binding energies of Mo and S in the MoS2 bilayer, we applied X-ray photoemission spectroscopy (XPS, K-alpha, Thermo VG) (Figure 1d). Two peaks, attributed to the doublet Mo 3d5/2 and Mo 3d3/2, are located at 229.4 and 232.5 eV. The other two peaks, corresponding to the S 2p3/2 and S 2p1/2 orbital of divalent sulfide ions (S2−), are observed at 162.3 and 163.5 eV. Those results indicate the existence of Mo4+ and S2−, with an atomic composition ratio for Mo and S of 1:2.[25,26] Furthermore, Figure 1e presents the Raman and photoluminescence (PL) spectra for the MoS2 bilayer (Uni-RAM2, UniNanoTech). The Raman characteristic peaks at 384 and 405 cm−1 correspond to the in-plane vibration of Mo and S atoms (E12g mode) and out-of-plane vibration of S atoms (A1g mode), respectively.[27] It is clear that the frequency difference (Δk) between the E12g and A1g modes is ≈21 cm−1, which is close to exfoliated bilayer MoS2.[28] In addition, the PL spectrum inserted in Figure 1e shows one main emission peak at 1.88 eV and confirms the characteristic of bilayer and good crystalline quality of the obtained MoS2. By precisely controlling the partial pressure of gaseous MoS2 species and the interaction of MoS2 thin films with SiO2 substrate, the synthetic process of MoS2 could follow the self-limiting mechanism, which is the main reason why the MoS2 can have uniform thickness.[24,29] A field effect transistor fabricated to confirm the electrical properties of the synthesized MoS2 film exhibits an on/off ratio of 107 and an average mobility of 0.51 ± 0.10 cm2 V−1 s−1 (Figure S1, Supporting Information). We fabricated a 4 × 4 array of MoS2 strain gauges on a large area (2.2 cm × 2.2 cm) to test the feasibility of this ultrathin, conformable MoS2-based device as a tactile sensor (Figure 1f).
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COMMUNICATION Figure 1. Fabrication and characterization of a wafer-scale continuous MoS2 bilayer. a) Schematic illustration of the synthesis of an MoS2 atomic layer by three-zone CVD. The inset shows optical images of the 4 cm × 3 cm SiO2 substrate before and after MoS2 deposition. b,c) Microscopic and AFM images of the as-grown MoS2 bilayer. d) X-ray photoemission spectroscopy scans for the Mo and S binding energies of the MoS2 bilayer. e) Raman spectrum of the obtained MoS2 bilayer. The inset shows its room temperature photoluminescence spectrum. f) Fabrication steps for the strain gauge sensor based on the MoS2.
First, we transferred graphene grown by the chemical vapor depostion (CVD) method to a SiO2/Si handling wafer coated with SU-8 epoxy and patterned to shape electrodes with interdigitated geometry and interconnects. Next, we printed MoS2 on the graphene electrodes and patterned it using photolithography and CHF3/O2 plasma (35/15 sccm, 100 W, 10 s). The area of a unit cell is 2.10 mm × 1.85 mm (interdigitated structure channel width (W): 37.9 mm, length (L): 0.05 mm). Then, we separated the device from the handling wafer to produce a freestanding film for fabrication of conformal devices on unusual
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substrates. To realize a highly conformal device, we reduced the total thickness to less than 75 nm by using bilayer MoS2 (1.4 nm), graphene (0.9 nm), SU-8 passivation layer (35 nm), and SU-8 substrate (35 nm) as the strain-sensing layer, the electrode, encapsulation layer, and the supporting layer, respectively. A key feature of atomically thin MoS2 is its high optical transmittance in the visible color range. Incorporation of conventional opaque metal electrodes into the MoS2 device could seriously attenuate its overall optical transmittance. In contrast, graphene has good optical transmittance and electrical
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conductivity as a sort of 2D material, like MoS2.[20,30–33] We transferred the three-layered graphene with optical transmittance of better than 93% in the visible range to the MoS2 channel region. The transmittance declined ≈6% in the MoS2 channel region and ≈13% at 550 nm in the MoS2/graphene overlapped region (Figure S2, Supporting Information). The quality of contact between MoS2 and graphene could influence sensor performances. We have observed I–V curves linear during a series of voltage sweeps from –1 V to 1 V, suggesting that an ohmic contact was made between MoS2 and graphene (Figure S3, Supporting Information). The work function and Fermi levels of graphene could be modulated by an unintentional electronic doping during fabrication processes, which leads to reduction in the barrier height at the MoS2/graphene interface.[22] We suspect such an effect plays a key role in exhibiting an outstanding ohmic contact in our experiments (Figure S4, Supporting Information). In addition, it should be noted that both materials have piezoresistivity, such that the measured resistance changes induced by external strains are contributions from not only MoS2 strain gauges but also graphene electrodes. However, the contribution of the MoS2 strain gauge dominates that of graphene electrodes by a factor of approximately five orders in magnitude due to large differences in
the resistance value and the GF between MoS2 and graphene (see the Supporting Information). To evaluate the piezoresistive effect of MoS2 strain gauges, we performed compressive/ tensile tests under different levels of strain on the thick polyethylene terephthalate substrate. Figure 2a shows the change in resistance of a MoS2 strain gauge with respect to a compressive strain produced by downward bending. The measured result can be divided into three regions : linear, nonlinear, and failure modes. In the linear section, below a strain of −1.98%, the MoS2 strain gauge exhibits reversible change in resistance. In the nonlinear region, corresponding to a strain ranging from −5.23% to −1.98%, it shows irreversible change in resistance (i.e., a hysteresis) (Figures S5 and S6, Supporting Information), which results from defects such as wrinkles and microcracks created in MoS2. We observed an abrupt increase in resistance at a strain of −5.23%, suggesting that the MoS2 film grown by CVD starts to macroscopically break at that strain. The limited mechanical properties of the sensor can be enhanced by adopting specific designs such as buckling and serpentine structure.[34,35] We conducted rigorous experiments to test the performance of the MoS2 strain gauges in the linear strain region. Compressive and tensile loads were conducted using a bending machine to induce strain in the MoS2 strain gauge
Figure 2. Electrical characterization of MoS2 strain gauge. a) The relative resistance changes of MoS2 strain gauge from −6% to 0% (failure, nonlinear and linear regions). b) The change in the relative resistances and gauge factors as a function of the applied tensile and compressive strain in the linear region (from −1.98% to +1.98%). c) The characteristic curves of the MoS2 tactile sensor as a function of time at various pressure levels with 20.0 kPa on the thick (thickness : 50.0 µm) and thin (thickness : 72.3 nm) substrates. The inset indicates the characteristics in a low range of pressure levels with 1.24 kPa steps on the thin substrate. d) The simulated strain mapping data of MoS2 tactile sensor on the thick and thin substrate at 20.0 kPa. e) The repeatable measurement data with a variety of pressure steps of 1.24 and 20.0 kPa, up to 1000 cycles.
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GF = ( ΔR / Ro) / ε
(1)
where Ro is the resistance at the initial state, ΔR is its change, and ε is the strain. In addition, our tactile sensor shows a good stability, indicating maximum resistance change of 3.24% under a compressive strain of 0.24% during a bending test for 10,000 cycles (Figure S7, Supporting Information). Furthermore, the sensor keeps its original properties at various repetitive strain loadings over 1000 cycles (Figure S8, Supporting Information) and exhibits low dependence on various strain rates (Figure S9, Supporting Information). GF of MoS2 is one order of magnitude larger than that of conventional metallic strain gauges (≈2–5) and several times higher than that of polycrystalline silicon (≈20–40).[36,37] However, it is lower than that of a previously reported single crystalline MoS2 strain gauge (≈230) because of its small grain size and the material imperfections associated with using the CVD process for large-area growth. Although our MoS2 strain gauges exhibit relatively lower GF than single crystal ones, the CVD method has the merit of large-area fabrication for various practical applications, such as E-skin. We believe that the improved growth process that synthesizes a large grain without creating defects enables us to
produce better strain sensors with GFs comparable to those of exfoliated sensors.[23] Figure 2c presents the outputs of a tactile sensor made of MoS2 strain gauges in response to vertical pressures from 0 to 100 kPa with an equal step of 20.0 kPa on thick (thickness 50.0 µm) and thin (thickness 72.3 nm) plastic substrates, and the inset indicates a response in a lower range of pressure levels with 1.24 kPa steps on a thin substrate. The minimum detectable pressure for our device (1.24 kPa) is comparable to the threshold values of human skin, which are in a range of 1–40 kPa.[38,39] The fractional change in resistance at various pressure levels with 20 kPa steps (or 1.24 kPa steps) between 0 and 100 kPa indicates that the MoS2-based tactile sensor has good resolution, low hysteresis, and low drift with time. Our tactile sensor had stably worked up to 200 kPa, which is assumed to be the maximum pressure for the grip control of object.[40] We observed a permanent failure at the pressure of 240 kPa (Figure S10, Supporting Information). It is obvious that the devices on a thin substrate exhibit much higher sensitivity to vertical pressure than those on a thick substrate because overall device stiffness with respect to vertical pressure depends greatly on the thickness of the substrate, as crosschecked by using the finite element method (FEM) shown in Figure 2d. At this point of view, the device strains on the thick substrate and the thin substrate at a pressure of 20.0 kPa are calculated to be 0.040% and 0.145% in the FEM simulation, respectively, and the corresponding fractional changes in resistance are estimated to be 2.90% and 10.5%, respectively, when the GF of the MoS2 strain gauge determined in Figure 2b is used. We experimentally measured the fractional changes in resistance on the thick and thin substrates as 2.03% and 9.78%, respectively, suggesting that our estimation is reasonably close to our experimental results. There might
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ranging from −1.98% to 1.98% with 36 steps in 16 cells. We recorded and averaged the relative resistance changes for each strain. As shown in Figure 2b, the relationship between the fractional resistance change and the strain is quite linear; the correlation coefficient of the linear fit (i.e., adjusted R-square) was evaluated to be 0.989. The GFs defined by Equation (1) were determined to be −72.5 ± 1.9 and −56.5 ± 4.8 under the compressive strain and the tensile strain, respectively (i.e., corresponding to the slope of the linear fit line)
Figure 3. High flexibility and transparency of MoS2 tactile sensor. a) Photographs of the MoS2 tactile sensor floated on the surface of water and transferred to wire (φ : 50 µm) and thin glass. b) Resistance variation of the MoS2 tactile sensor during repeated deformations: twist angle (20°) and bending radius (0.6 cm). The inset shows a photograph of the MoS2 tactile sensor on leather (roughness : 150 µm).
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Figure 4. Pressure mapping of MoS2 tactile sensor conformally transferred on the fingertip. a) Photographs of the MoS2 tactile sensor on a fingertip. b) Surface profiles of the fingerprint with the tactile sensor. The inset shows an SEM image of the fingerprint (scale bar : 1 mm). c) Colorized SEM images of the MoS2 strain gauge (left, yellow) and graphene electrode (right, blue). d) Pressure map detected by the MoS2 tactile sensor from contact with pens of different diameters (7 and 5 mm). e) Maximum fractional resistance change of the MoS2 sensor array with different pen-tip sizes (D: 7, 5, and 2 mm). f) Tracking results for the position of a moving, touching pen (D: 2 mm) on the tactile sensor.
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We mapped the pressure distributions of the fingertip using the MoS2 tactile sensor pressed by stylus pens of three different diameters (2, 5, and 7 mm) with the same amount of forces, as shown in Figure 4d. When the pen-tip size is small, the stress is concentrated on a specific area of the fingertip, whereas a pen with a large tip distributes stress over a large area, resulting in a low strain value. The magnitude of the maximum fractional resistance change measured by the MoS2 tactile sensor increases from 3.4% to 8.0% as the pen size decreases (Figure 4e). The position of a pen (D: 2 mm) moving quickly on the tactile sensor was well tracked without creation of crack or delamination of the device (Figure 4f) because of the strong adhesion forces that originated from the conformal contact and the high sensitivity of the MoS2 strain gauges. In addition, the MoS2 tactile sensor could be capable of measuring multidirectional forces when employing a special sensing structure such as a microdome or a bump along with an adequate strain gauge layout.[11,45,46] The resistance of MoS2 strain gauges is affected by not only strain but also temperature. We observed a relative resistance change of 14.2% ± 1.2% in response to a temperature change from 30 to 40 °C, which corresponds to the temperature range of human skin (Figure S11, Supporting Information). Such a temperature dependency can be compensated using a Wheatstone bridge configuration or using a dummy MoS2 strain gauge attached on the place where it is affected by only temperature, not by strain (e.g., a nail area). It is also significant to reduce degradation of the device induced by an absorption of ionic and water molecules produced from sweat. The absorption of electron-withdrawing/donating molecules causes the change in resistance of MoS2. The passivation layers with hydrophobic properties such as SU-8 epoxy can help to reduce the effect of water and sweat on the sensor (Figure S12, Supporting Information). In summary, we demonstrated a large-area, conformal tactile sensor fabricated with a MoS2 semiconductor and graphene electrodes grown by CVD. The MoS2 strain sensor exhibited a variety of advantages, such as good optical transparency, mechanical flexibility, and high GF, compared with conventional strain gauges. In particular, ultrathin nature of the device (≈75 nm) enabled us to create it on unusual substrates, such as a fingertip or leather products. Furthermore, this prototype of the conformal MoS2 tactile sensor can be utilized to achieve a high integration density array, low crosstalk, and high switching speed through active matrix circuitry. We expect its benefits to make the MoS2-based sensors useful for wearable applications, such as E-skin.
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be a tradeoff between capacity (i.e., the maximum allowable pressure) and resolution (i.e., the minimum detectable pressure) when determining the thickness of the substrate. As the substrate gets thinner, the sensitivity to pressure gets higher, but strain could be induced above the limits of the linear range (i.e., ±1.98%) at a lower pressure. Fractional resistance changes maintained their initial values over 1000 loading cycles at various pressure levels with 1.24 kPa and 20.0 kPa steps, as shown in Figure 2e, suggesting that the MoS2-based tactile sensing device has a low repeatable error. Here, it is worth noting that we observed a small amount of drift in fractional resistance changes over cycles, ranging from 5% to 10%, which we also observed (but much smaller) during the cyclic bending test (Figures S7 and S8, Supporting Information). The drift may originate from the gradual mechanical destruction of the MoS2 strain gauges by direct and repeated physical contact on the thin gauges. An introduction of additional passivation layer on the strain gauges is expected to protect the devices from such mechanical destruction. In addition to their high sensitivity, ultrathin MoS2 tactile sensing devices with nanometer-scale total thickness can achieve conformal contact to substrates with arbitrary surface shapes because van der Waals forces are sufficient to stick them on the substrates.[41–44] Figure 3a shows an extremely mechanically flexible and transparent 75 nm thick MoS2 device released from a handling substrate. It can freely float on the surface of the solution and even be folded onto the edge of a thin wire with 50 µm diameter. MoS2 strain gauges and graphene interconnects were hardly visible in the photographs due to their excellent transparency (Figure S2, Supporting Information). Figure 3b presents variations of fractional resistance with the device on leather with a surface roughness of more than 150 µm in response to repeated twisting and bending motions. Fractional resistance measured at four different loading states (i.e., ∞, 0.6 cm bending radii, 0 and 20° twisting loadings) varied over 250 cycles with relative deviations smaller than 9.7% and 10.1% for the bending and twisting states, respectively, which implies that our device operated stably without serious degradation. This could be explained as follows: atomically thin MoS2 and graphene possess good mechanical bendability, and three-dimensionally curved leather surfaces can absorb most of the external strain through transformation and flattening of the surface shapes without delivering the whole strain to the device adhered conformably to it.[42] The presented devices with outstanding mechanical properties can be used for various E-skin applications. To demonstrate an example of E-skin applications, we fabricated a tactile sensor with a 4 × 4 array of MoS2 strain gauges on a fingertip (Figure 4a). The dark squares and the lines on the fingertip in the photo indicate MoS2 channels and graphene interconnects. The device printed on the fingertip is hardly recognizable with the bare eyes because of its good optical transparency. We confirmed that the whole device, consisting of the MoS2 strain gauges and graphene interconnects, could conformably cover fingerprint ridges with surface roughness: Ra = 40 µm through a profile measurement (Figure 4b) and fieldemission scanning electron microscope (FE-SEM) images (Figure 4c).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements M.P. and Y.J.P. contributed equally to this work. This work was supported by the Center for Advanced Soft-Electronics funded by the
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Ministry of Science, ICT, and Future Planning as Global Frontier Project (CASE2014M3A6A5060933), the National Creative Research Laboratory (2015R1A3A2066337) through the National Research Foundation of Korea (NRF), and the ICT R&D program of MSIP/IITP (R0101-15-0034). Received: October 17, 2015 Revised: December 9, 2015 Published online: February 2, 2016
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MoS2-Based Tactile Sensor for Electronic Skin Applications Minhoon Park, Yong Ju Park, Xiang Chen, Yon-Kyu Park, Min-Seok Kim,* and Jong-Hyun Ahn* Tactile sensors, in the form of conformal and embedded devices, have attracted intense research interest because of their diverse applications, from electronic skin (E-skin) for robotics to health care monitoring systems. Much effort has been made to develop large-area and high-performance tactile sensors with good sensitivity and mechanical flexibility. To address those issues, various strain sensing materials have been studied. Examples include nanomaterials, such as graphene and carbon nanotubes, and hybrid composites, such as metal particles-polymers and conductive materials-nanostructure polymers.[1–11] Despite high sensitivity with respect to external strain, existing sensors have several drawbacks: high hysteresis, nonlinearity, and poor repeatability.[12,13] In contrast, semiconductor material-based piezoresistive sensors give high sensitivity and stable operation.[14–16] As an alternative, MoS2 semiconductors have recently attracted attention because of their outstanding mechanical and optical transmittance, high gauge factor (GF), and tunable band gap.[17–23] However, no one has yet reported on multicell devices for practical applications that transcend the fabrication of a single cell. Here, we present an ultrathin conformal, MoS2-based tactile sensing array covering an area of 2.2 cm × 2.2 cm. We integrated the sensor with a graphene electrode and interconnect to achieve good mechanical flexibility and optical transmittance in the visible color range. This sensor shows high sensitivity, good uniformity, and linearity even after 10,000 loading cycles. In addition, it provides excellent mechanical flexibility over a strain of 1.98% and good optical transparency over 80%. Our ultrathin tactile sensor fabricated on an ultrathin, transparent plastic substrate presented stable performance even on unusual substrates such as leather and a human fingertip. Figure 1a presents a schematic illustration of a 2 in. quartz tube furnace with three temperature zones for the synthesis of wafer-scale, thin-layer MoS2. We placed 0.5–1.5 g of sulfur (S) and 20–70 mg of molybdenum oxide (MoO3) powders (99.5%, Sigma-Aldrich) in two small quartz tubes separately at the upstream end and 4 cm × 3 cm of SiO2 (300 nm)/Si substrate
M. Park, Y. J. Park, Dr. X. Chen, Prof. J.-H. Ahn School of Electrical and Electronic Engineering Yonsei University 50 Yonsei-ro Seodaemun-gu, Seoul 03722, Republic of Korea E-mail: [email protected] M. Park, Dr. Y.-K. Park, Dr. M.-S. Kim Center for Mass and Related Quantities Korea Research Institute of Standards and Science 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea E-mail: [email protected]
DOI: 10.1002/adma.201505124
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at the downstream end. The setting temperatures for zone I (S powder), zone II (MoO3 powder), and zone III (SiO2/Si substrate) were 110–150 °C, 580– 650 °C, and 650–750 °C, respectively. During the reaction process, we kept the chamber at a low pressure (0.7 Torr) in an Ar atmosphere (flow rate: 100 sccm). After 30–60 min, we turned the furnace off, moved it away, and left the sample in the chamber to cool slowly to room temperature. Compared with the bare SiO2/Si substrate, the color of the as-grown MoS2 sample obviously changed from purple to blue (inset of Figure 1a). The optical microscopic image of the MoS2 layer on SiO2/Si substrate reveals a uniform and continuous color distribution across the wafer (Figure 1b). To estimate the thickness of the synthesized MoS2 layer, we used atomic force microscopy (AFM, XE-100, Park system) to analyze the topography of the area near the border (Figure 1c). The inset is the cross-sectional height profile for the trace, indicated as a red line. The result shows that the thickness of the MoS2 layer is ≈1.4 nm, indicating a bilayer.[24] We used this bilayer MoS2 because it showed the most reasonable characteristics in terms of electrical and optical properties. To measure the binding energies of Mo and S in the MoS2 bilayer, we applied X-ray photoemission spectroscopy (XPS, K-alpha, Thermo VG) (Figure 1d). Two peaks, attributed to the doublet Mo 3d5/2 and Mo 3d3/2, are located at 229.4 and 232.5 eV. The other two peaks, corresponding to the S 2p3/2 and S 2p1/2 orbital of divalent sulfide ions (S2−), are observed at 162.3 and 163.5 eV. Those results indicate the existence of Mo4+ and S2−, with an atomic composition ratio for Mo and S of 1:2.[25,26] Furthermore, Figure 1e presents the Raman and photoluminescence (PL) spectra for the MoS2 bilayer (Uni-RAM2, UniNanoTech). The Raman characteristic peaks at 384 and 405 cm−1 correspond to the in-plane vibration of Mo and S atoms (E12g mode) and out-of-plane vibration of S atoms (A1g mode), respectively.[27] It is clear that the frequency difference (Δk) between the E12g and A1g modes is ≈21 cm−1, which is close to exfoliated bilayer MoS2.[28] In addition, the PL spectrum inserted in Figure 1e shows one main emission peak at 1.88 eV and confirms the characteristic of bilayer and good crystalline quality of the obtained MoS2. By precisely controlling the partial pressure of gaseous MoS2 species and the interaction of MoS2 thin films with SiO2 substrate, the synthetic process of MoS2 could follow the self-limiting mechanism, which is the main reason why the MoS2 can have uniform thickness.[24,29] A field effect transistor fabricated to confirm the electrical properties of the synthesized MoS2 film exhibits an on/off ratio of 107 and an average mobility of 0.51 ± 0.10 cm2 V−1 s−1 (Figure S1, Supporting Information). We fabricated a 4 × 4 array of MoS2 strain gauges on a large area (2.2 cm × 2.2 cm) to test the feasibility of this ultrathin, conformable MoS2-based device as a tactile sensor (Figure 1f).
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COMMUNICATION Figure 1. Fabrication and characterization of a wafer-scale continuous MoS2 bilayer. a) Schematic illustration of the synthesis of an MoS2 atomic layer by three-zone CVD. The inset shows optical images of the 4 cm × 3 cm SiO2 substrate before and after MoS2 deposition. b,c) Microscopic and AFM images of the as-grown MoS2 bilayer. d) X-ray photoemission spectroscopy scans for the Mo and S binding energies of the MoS2 bilayer. e) Raman spectrum of the obtained MoS2 bilayer. The inset shows its room temperature photoluminescence spectrum. f) Fabrication steps for the strain gauge sensor based on the MoS2.
First, we transferred graphene grown by the chemical vapor depostion (CVD) method to a SiO2/Si handling wafer coated with SU-8 epoxy and patterned to shape electrodes with interdigitated geometry and interconnects. Next, we printed MoS2 on the graphene electrodes and patterned it using photolithography and CHF3/O2 plasma (35/15 sccm, 100 W, 10 s). The area of a unit cell is 2.10 mm × 1.85 mm (interdigitated structure channel width (W): 37.9 mm, length (L): 0.05 mm). Then, we separated the device from the handling wafer to produce a freestanding film for fabrication of conformal devices on unusual
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substrates. To realize a highly conformal device, we reduced the total thickness to less than 75 nm by using bilayer MoS2 (1.4 nm), graphene (0.9 nm), SU-8 passivation layer (35 nm), and SU-8 substrate (35 nm) as the strain-sensing layer, the electrode, encapsulation layer, and the supporting layer, respectively. A key feature of atomically thin MoS2 is its high optical transmittance in the visible color range. Incorporation of conventional opaque metal electrodes into the MoS2 device could seriously attenuate its overall optical transmittance. In contrast, graphene has good optical transmittance and electrical
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conductivity as a sort of 2D material, like MoS2.[20,30–33] We transferred the three-layered graphene with optical transmittance of better than 93% in the visible range to the MoS2 channel region. The transmittance declined ≈6% in the MoS2 channel region and ≈13% at 550 nm in the MoS2/graphene overlapped region (Figure S2, Supporting Information). The quality of contact between MoS2 and graphene could influence sensor performances. We have observed I–V curves linear during a series of voltage sweeps from –1 V to 1 V, suggesting that an ohmic contact was made between MoS2 and graphene (Figure S3, Supporting Information). The work function and Fermi levels of graphene could be modulated by an unintentional electronic doping during fabrication processes, which leads to reduction in the barrier height at the MoS2/graphene interface.[22] We suspect such an effect plays a key role in exhibiting an outstanding ohmic contact in our experiments (Figure S4, Supporting Information). In addition, it should be noted that both materials have piezoresistivity, such that the measured resistance changes induced by external strains are contributions from not only MoS2 strain gauges but also graphene electrodes. However, the contribution of the MoS2 strain gauge dominates that of graphene electrodes by a factor of approximately five orders in magnitude due to large differences in
the resistance value and the GF between MoS2 and graphene (see the Supporting Information). To evaluate the piezoresistive effect of MoS2 strain gauges, we performed compressive/ tensile tests under different levels of strain on the thick polyethylene terephthalate substrate. Figure 2a shows the change in resistance of a MoS2 strain gauge with respect to a compressive strain produced by downward bending. The measured result can be divided into three regions : linear, nonlinear, and failure modes. In the linear section, below a strain of −1.98%, the MoS2 strain gauge exhibits reversible change in resistance. In the nonlinear region, corresponding to a strain ranging from −5.23% to −1.98%, it shows irreversible change in resistance (i.e., a hysteresis) (Figures S5 and S6, Supporting Information), which results from defects such as wrinkles and microcracks created in MoS2. We observed an abrupt increase in resistance at a strain of −5.23%, suggesting that the MoS2 film grown by CVD starts to macroscopically break at that strain. The limited mechanical properties of the sensor can be enhanced by adopting specific designs such as buckling and serpentine structure.[34,35] We conducted rigorous experiments to test the performance of the MoS2 strain gauges in the linear strain region. Compressive and tensile loads were conducted using a bending machine to induce strain in the MoS2 strain gauge
Figure 2. Electrical characterization of MoS2 strain gauge. a) The relative resistance changes of MoS2 strain gauge from −6% to 0% (failure, nonlinear and linear regions). b) The change in the relative resistances and gauge factors as a function of the applied tensile and compressive strain in the linear region (from −1.98% to +1.98%). c) The characteristic curves of the MoS2 tactile sensor as a function of time at various pressure levels with 20.0 kPa on the thick (thickness : 50.0 µm) and thin (thickness : 72.3 nm) substrates. The inset indicates the characteristics in a low range of pressure levels with 1.24 kPa steps on the thin substrate. d) The simulated strain mapping data of MoS2 tactile sensor on the thick and thin substrate at 20.0 kPa. e) The repeatable measurement data with a variety of pressure steps of 1.24 and 20.0 kPa, up to 1000 cycles.
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GF = ( ΔR / Ro) / ε
(1)
where Ro is the resistance at the initial state, ΔR is its change, and ε is the strain. In addition, our tactile sensor shows a good stability, indicating maximum resistance change of 3.24% under a compressive strain of 0.24% during a bending test for 10,000 cycles (Figure S7, Supporting Information). Furthermore, the sensor keeps its original properties at various repetitive strain loadings over 1000 cycles (Figure S8, Supporting Information) and exhibits low dependence on various strain rates (Figure S9, Supporting Information). GF of MoS2 is one order of magnitude larger than that of conventional metallic strain gauges (≈2–5) and several times higher than that of polycrystalline silicon (≈20–40).[36,37] However, it is lower than that of a previously reported single crystalline MoS2 strain gauge (≈230) because of its small grain size and the material imperfections associated with using the CVD process for large-area growth. Although our MoS2 strain gauges exhibit relatively lower GF than single crystal ones, the CVD method has the merit of large-area fabrication for various practical applications, such as E-skin. We believe that the improved growth process that synthesizes a large grain without creating defects enables us to
produce better strain sensors with GFs comparable to those of exfoliated sensors.[23] Figure 2c presents the outputs of a tactile sensor made of MoS2 strain gauges in response to vertical pressures from 0 to 100 kPa with an equal step of 20.0 kPa on thick (thickness 50.0 µm) and thin (thickness 72.3 nm) plastic substrates, and the inset indicates a response in a lower range of pressure levels with 1.24 kPa steps on a thin substrate. The minimum detectable pressure for our device (1.24 kPa) is comparable to the threshold values of human skin, which are in a range of 1–40 kPa.[38,39] The fractional change in resistance at various pressure levels with 20 kPa steps (or 1.24 kPa steps) between 0 and 100 kPa indicates that the MoS2-based tactile sensor has good resolution, low hysteresis, and low drift with time. Our tactile sensor had stably worked up to 200 kPa, which is assumed to be the maximum pressure for the grip control of object.[40] We observed a permanent failure at the pressure of 240 kPa (Figure S10, Supporting Information). It is obvious that the devices on a thin substrate exhibit much higher sensitivity to vertical pressure than those on a thick substrate because overall device stiffness with respect to vertical pressure depends greatly on the thickness of the substrate, as crosschecked by using the finite element method (FEM) shown in Figure 2d. At this point of view, the device strains on the thick substrate and the thin substrate at a pressure of 20.0 kPa are calculated to be 0.040% and 0.145% in the FEM simulation, respectively, and the corresponding fractional changes in resistance are estimated to be 2.90% and 10.5%, respectively, when the GF of the MoS2 strain gauge determined in Figure 2b is used. We experimentally measured the fractional changes in resistance on the thick and thin substrates as 2.03% and 9.78%, respectively, suggesting that our estimation is reasonably close to our experimental results. There might
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ranging from −1.98% to 1.98% with 36 steps in 16 cells. We recorded and averaged the relative resistance changes for each strain. As shown in Figure 2b, the relationship between the fractional resistance change and the strain is quite linear; the correlation coefficient of the linear fit (i.e., adjusted R-square) was evaluated to be 0.989. The GFs defined by Equation (1) were determined to be −72.5 ± 1.9 and −56.5 ± 4.8 under the compressive strain and the tensile strain, respectively (i.e., corresponding to the slope of the linear fit line)
Figure 3. High flexibility and transparency of MoS2 tactile sensor. a) Photographs of the MoS2 tactile sensor floated on the surface of water and transferred to wire (φ : 50 µm) and thin glass. b) Resistance variation of the MoS2 tactile sensor during repeated deformations: twist angle (20°) and bending radius (0.6 cm). The inset shows a photograph of the MoS2 tactile sensor on leather (roughness : 150 µm).
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Figure 4. Pressure mapping of MoS2 tactile sensor conformally transferred on the fingertip. a) Photographs of the MoS2 tactile sensor on a fingertip. b) Surface profiles of the fingerprint with the tactile sensor. The inset shows an SEM image of the fingerprint (scale bar : 1 mm). c) Colorized SEM images of the MoS2 strain gauge (left, yellow) and graphene electrode (right, blue). d) Pressure map detected by the MoS2 tactile sensor from contact with pens of different diameters (7 and 5 mm). e) Maximum fractional resistance change of the MoS2 sensor array with different pen-tip sizes (D: 7, 5, and 2 mm). f) Tracking results for the position of a moving, touching pen (D: 2 mm) on the tactile sensor.
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We mapped the pressure distributions of the fingertip using the MoS2 tactile sensor pressed by stylus pens of three different diameters (2, 5, and 7 mm) with the same amount of forces, as shown in Figure 4d. When the pen-tip size is small, the stress is concentrated on a specific area of the fingertip, whereas a pen with a large tip distributes stress over a large area, resulting in a low strain value. The magnitude of the maximum fractional resistance change measured by the MoS2 tactile sensor increases from 3.4% to 8.0% as the pen size decreases (Figure 4e). The position of a pen (D: 2 mm) moving quickly on the tactile sensor was well tracked without creation of crack or delamination of the device (Figure 4f) because of the strong adhesion forces that originated from the conformal contact and the high sensitivity of the MoS2 strain gauges. In addition, the MoS2 tactile sensor could be capable of measuring multidirectional forces when employing a special sensing structure such as a microdome or a bump along with an adequate strain gauge layout.[11,45,46] The resistance of MoS2 strain gauges is affected by not only strain but also temperature. We observed a relative resistance change of 14.2% ± 1.2% in response to a temperature change from 30 to 40 °C, which corresponds to the temperature range of human skin (Figure S11, Supporting Information). Such a temperature dependency can be compensated using a Wheatstone bridge configuration or using a dummy MoS2 strain gauge attached on the place where it is affected by only temperature, not by strain (e.g., a nail area). It is also significant to reduce degradation of the device induced by an absorption of ionic and water molecules produced from sweat. The absorption of electron-withdrawing/donating molecules causes the change in resistance of MoS2. The passivation layers with hydrophobic properties such as SU-8 epoxy can help to reduce the effect of water and sweat on the sensor (Figure S12, Supporting Information). In summary, we demonstrated a large-area, conformal tactile sensor fabricated with a MoS2 semiconductor and graphene electrodes grown by CVD. The MoS2 strain sensor exhibited a variety of advantages, such as good optical transparency, mechanical flexibility, and high GF, compared with conventional strain gauges. In particular, ultrathin nature of the device (≈75 nm) enabled us to create it on unusual substrates, such as a fingertip or leather products. Furthermore, this prototype of the conformal MoS2 tactile sensor can be utilized to achieve a high integration density array, low crosstalk, and high switching speed through active matrix circuitry. We expect its benefits to make the MoS2-based sensors useful for wearable applications, such as E-skin.
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be a tradeoff between capacity (i.e., the maximum allowable pressure) and resolution (i.e., the minimum detectable pressure) when determining the thickness of the substrate. As the substrate gets thinner, the sensitivity to pressure gets higher, but strain could be induced above the limits of the linear range (i.e., ±1.98%) at a lower pressure. Fractional resistance changes maintained their initial values over 1000 loading cycles at various pressure levels with 1.24 kPa and 20.0 kPa steps, as shown in Figure 2e, suggesting that the MoS2-based tactile sensing device has a low repeatable error. Here, it is worth noting that we observed a small amount of drift in fractional resistance changes over cycles, ranging from 5% to 10%, which we also observed (but much smaller) during the cyclic bending test (Figures S7 and S8, Supporting Information). The drift may originate from the gradual mechanical destruction of the MoS2 strain gauges by direct and repeated physical contact on the thin gauges. An introduction of additional passivation layer on the strain gauges is expected to protect the devices from such mechanical destruction. In addition to their high sensitivity, ultrathin MoS2 tactile sensing devices with nanometer-scale total thickness can achieve conformal contact to substrates with arbitrary surface shapes because van der Waals forces are sufficient to stick them on the substrates.[41–44] Figure 3a shows an extremely mechanically flexible and transparent 75 nm thick MoS2 device released from a handling substrate. It can freely float on the surface of the solution and even be folded onto the edge of a thin wire with 50 µm diameter. MoS2 strain gauges and graphene interconnects were hardly visible in the photographs due to their excellent transparency (Figure S2, Supporting Information). Figure 3b presents variations of fractional resistance with the device on leather with a surface roughness of more than 150 µm in response to repeated twisting and bending motions. Fractional resistance measured at four different loading states (i.e., ∞, 0.6 cm bending radii, 0 and 20° twisting loadings) varied over 250 cycles with relative deviations smaller than 9.7% and 10.1% for the bending and twisting states, respectively, which implies that our device operated stably without serious degradation. This could be explained as follows: atomically thin MoS2 and graphene possess good mechanical bendability, and three-dimensionally curved leather surfaces can absorb most of the external strain through transformation and flattening of the surface shapes without delivering the whole strain to the device adhered conformably to it.[42] The presented devices with outstanding mechanical properties can be used for various E-skin applications. To demonstrate an example of E-skin applications, we fabricated a tactile sensor with a 4 × 4 array of MoS2 strain gauges on a fingertip (Figure 4a). The dark squares and the lines on the fingertip in the photo indicate MoS2 channels and graphene interconnects. The device printed on the fingertip is hardly recognizable with the bare eyes because of its good optical transparency. We confirmed that the whole device, consisting of the MoS2 strain gauges and graphene interconnects, could conformably cover fingerprint ridges with surface roughness: Ra = 40 µm through a profile measurement (Figure 4b) and fieldemission scanning electron microscope (FE-SEM) images (Figure 4c).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements M.P. and Y.J.P. contributed equally to this work. This work was supported by the Center for Advanced Soft-Electronics funded by the
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Ministry of Science, ICT, and Future Planning as Global Frontier Project (CASE2014M3A6A5060933), the National Creative Research Laboratory (2015R1A3A2066337) through the National Research Foundation of Korea (NRF), and the ICT R&D program of MSIP/IITP (R0101-15-0034). Received: October 17, 2015 Revised: December 9, 2015 Published online: February 2, 2016
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