All-Graphene-Based Highly Flexible Noncontact Electronic Skin - ACS

Dec 5, 2017 - Department of Mechanical Engineering, Khalifa University of Science, Technology and Research, Abu Dhabi 127788, United Arab Emirates...
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All-Graphene-Based Highly Flexible Noncontact Electronic Skin Jianing An,† Truong-Son Dinh Le,†,‡ Yi Huang,† Zhaoyao Zhan,† Yong Li,† Lianxi Zheng,§ Wei Huang,∥ Gengzhi Sun,*,∥ and Young-Jin Kim*,† †

School of Mechanical and Aerospace Engineering, ‡Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore § Department of Mechanical Engineering, Khalifa University of Science, Technology and Research, Abu Dhabi 127788, United Arab Emirates ∥ Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China S Supporting Information *

ABSTRACT: Noncontact electronic skin (e-skin), which possesses superior long-range and high-spatial-resolution sensory properties, is becoming indispensable in fulfilling the emulation of human sensation via prosthetics. Here, we present an advanced design and fabrication of all-graphene-based highly flexible noncontact e-skins by virtue of femtosecond laser direct writing (FsLDW). The photoreduced graphene oxide patterns function as the conductive electrodes, whereas the pristine graphene oxide thin film serves as the sensing layer. The asfabricated e-skins exhibit high sensitivity, fast response−recovery behavior, good long-term stability, and excellent mechanical robustness. In-depth analysis reveals that the sensing mechanism is attributed to proton and ionic conductivity in the low and high humidity conditions, respectively. By taking the merits of the FsLDW, a 4 × 4 sensing matrix is facilely integrated in a single-step, eco-friendly, and green process. The light-weight and inplane matrix shows high-spatial-resolution sensing capabilities over a long detection range in a noncontact mode. This study will open up an avenue to innovations in the noncontact e-skins and hold a promise for applications in wearable human−machine interfaces, robotics, and bioelectronics. KEYWORDS: electronic skins, noncontact operation mode, graphene, flexible devices, femtosecond laser direct writing

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INTRODUCTION As the largest organ of the human body, skin possesses excellent sensory capabilities toward various environmental stimuli, thus mediates the human−environment interactions and acts as the outer barrier for the protection of human body. For people with skin damages or amputations, the restoration of the sensational properties of the skin via electronics delivers good healthcare, hence improves the quality of their lives.1,2 In recent years, there has been a surge in innovating flexible artificial skins that can be conformably adhered to human bodies. Various tactile sensors exploiting the transduction mechanisms of piezoresistivity, capacitance, contact resistance, and piezoelectricity have been intensively developed for the efficient detection and spatial mapping of the distribution of mechanical stimuli.3−10 The tactile sensing systems, however, could only discern the information acquired by physical contacts, which may generate critical safety issues because of the insufficient time for people to react when they are exposed to the thermally, chemically, and electrically harsh environment. Furthermore, the repetitive physical contacts also tend to © XXXX American Chemical Society

dramatically shorten the lifetime of the devices. By contrast, the electronic skins (e-skins) operated in noncontact modes can overcome the aforementioned limitations.11−13 For instance, the noncontact e-skins enable remote perception and distinction of external objects, thereby protecting the human body from dangerous sensing situations based on physical contacts. Besides, conferring touchless sensory capabilities to eskins also improves the durability of the devices by obviating the potential contaminations or damages owing to physical contacts. Therefore, noncontact sensing systems are becoming indispensable and complementary to the traditional contactbased sensors in fully imitating human sensation through prosthetic e-skins. Mimicry of the touchless sensing capabilities of human skin has been accessed by developing advanced sensory devices responding to stimuli such as temperature, photoelectric signal, Received: September 9, 2017 Accepted: December 5, 2017 Published: December 5, 2017 A

DOI: 10.1021/acsami.7b13701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Prototype demonstration of an e-skin mimicking the noncontact sensing properties of human skin. (a) Conceptual image of a flexible sensing matrix that conformably contacts with a human hand and provides viable responses to moisture stimuli. (b) Schematic plot of the relative change in impedance (ΔZ/Z0) of each pixel in the matrix, demonstrating the mapping capability of the e-skin in the noncontact mode.

Figure 2. Fabrication of the all-graphene, flexible, and in-plane noncontact sensors via programmable FsLDW system. (a) Schematic image of the FsLDW system for the single-step fabrication of all-graphene noncontact sensors. (b) Optical images (in flat and bent conditions) of the sensor where the rGO electrodes appear in black and the brown thin film corresponds to the GO sensing material. (c) Top-view and (d) tilted-view SEM images of the pristine GO and femtosecond laser-irradiated rGO thin films. (e) Cross-sectional SEM image of the stacked GO layers. (f) Crosssectional SEM image of the porous rGO layers.

and the flexible substrates seriously degrades the performance of the sensing systems, posing problems in e-skins following the dynamic motions of human body.11,17 To address the above-mentioned problems of noncontact eskins, here, we present an all-graphene-based flexible moisturesensing system produced by a programmable femtosecond laser direct writing (FsLDW) technique. Graphene oxide (GO), a green, biocompatible, solution-processable, and mass producible material, is used as the precursor.18,19 The oxygencontaining groups (OCGs) decorated on GO endow it with hydrophilicity and ionic conductivity. Although GO is an electrical insulator, its electrical conductivity can be readily restored through thermal reduction, chemical reduction, and photoreduction.20−24 The as-fabricated devices, which feature the photoreduced GO (rGO) interdigitated patterns and the pristine GO thin films as the electrodes and sensing material, respectively, demonstrate noncontact sensing capabilities with high sensitivity, fast response, rapid recovery, good long-term stability, and excellent mechanical flexibility and durability. By virtue of the designable and scalable writing capacity of

and moisture.11−15 Temperature sensors are of high significance in detecting the nearby objects without physical contacts based on simple thermal transport between the human body and the surrounding environment. However, the response time to thermal stimuli is limited by the slow heat-transfer process, and the measurement accuracy is relatively poor because of the adverse thermal disturbances from the background.15 Although photoelectric sensors have shown the potential for proximity sensing, a large interaction area and a short working distance are strictly required for achieving a good sensitivity.13 In comparison, the rapid diffusive and permeable characteristics of moisture make it an attractive long-range signal source for noncontact sensing.16 However, severe issues in terms of low stability and slow recovery exist in most precedent noncontact moisture e-skins, which greatly restrict their applications in prosthetics.5,15,16 In addition, the cumbersome fabrication processes and complicated multistep integration have led the production of noncontact flexible e-skins to be complex, lowyield, time-consuming, and costly.15 Moreover, the easy delamination between the nondeformable metallic electrodes B

DOI: 10.1021/acsami.7b13701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. Characterizations of the photoreduction of GO by controllable femtosecond laser irradiation. (a) Three-dimensional (3D) plot representing Rs of rGO resulted from varied laser writing parameters. (b) UV−vis spectra of GO and rGO thin films. (c) FTIR spectra of GO and rGO thin films. (d) XRD patterns of GO and rGO thin films. (e) Raman spectra of GO and rGO thin films. (f) C 1s XPS spectra of GO and rGO thin films.

thickness of rGO was increased compared with that of GO (Figure 2d). The cross-sectional SEM images (Figure 2e,f) clearly show that the stacked GO layers were converted to the porous rGO architectures after laser irradiation. The reduction mechanism of GO can be attributed to a combination of the electronic excitation effect upon femtosecond laser irradiation and the thermal effect owing to the subsequent electron−hole recombination.23 As the FsLDW technique allows for the arbitrary patterning on GO films following the predesigned features, different sample patterns, including comb, spiral, and continuous square patterns, were successfully manufactured as shown in Figure S1 (Supporting Information). In addition to the arbitrary patterning, FsLDW newly enables minimal damage (compared to the laser writing process based on continuous-wave lasers or long-pulse lasers) to the flexible substrates by minimizing the thermal effects in the substrates via optimizing the center wavelength and the pulse duration of the laser. This characteristic is essential in the fabrication of flexible noncontact e-skins, because the polymer-based materials that are known to be easily damaged by heat energy are normally adopted as the flexible substrates. In this study, a near-infrared (NIR) laser was utilized, at which wavelength (780 nm) the GO/rGO thin films exhibited a relatively high absorption coefficient compared with the flexible PET substrates. Therefore, the residual transmitted light from the GO/rGO passed through flexible substrates with a negligible absorption, avoiding thermal damages to the substrates. Furthermore, the pulse duration of the femtosecond laser (10−13 to 10−15 s) was shorter than the time required for the heat transfer in GO/rGO or substrates, hence the FsLDW process was confined into a small volume only without any thermal side effects to nearby materials. Since the electrical conductivity of GO could be readily restored by the FsLDW process,25−27 a series of rGO square

FsLDW, a single-step integration of the e-skin matrix is achieved in a mask-free, time-efficient, and cost-effective fashion.23,24 The in-plane, light-weight, and mechanically robust-sensing matrix exhibits a high spatial resolution over a long working range, holding an exceptional promise for noncontact e-skins.

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RESULTS AND DISCUSSION Figure 1a shows a conceptual image of the all-graphene, flexible, and in-plane e-skin, which is in conformal contact with a human hand and mimics the noncontact-sensing functionality of the human skin. When the sensing matrix is exposed to moisture stimuli, such as human breath or a human finger, the impedance of each pixel in the matrix decreases accordingly, as schematically plotted in Figure 1b. Prominent impedance changes can be observed from the pixels located close to the stimuli, whereas the other pixels distant from the stimuli provide much weaker responses. The current position and moving direction of the stimuli can be spatially mapped by the noncontact e-skin. A programmable FsLDW system (Figure 2a) was developed to realize the direct writing of the all-graphene in-plane sensor comprising patterned rGO as the electrodes and pristine GO as the sensing material. Following the fabrication procedures described in the methods section, the focused femtosecond laser beam was directed on the GO thin film (deposited on the polyethylene terephthalate (PET) substrate) printing the preprogrammed interdigitated patterns. The laser-induced spatial patterning and the synchronous photoreduction converted GO into rGO, resulting in a color change from yellow-brown to black (Figure 2b). Morphology change of the thin film upon femtosecond laser irradiation was characterized by scanning electron microscopy (SEM). After laser irradiation, the rGO flakes were formed as shown in Figure 2c, and the C

DOI: 10.1021/acsami.7b13701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Sensing properties, sensing mechanism, and performance durability of the as-fabricated sensor. (a) Plots of impedance as a function of RH at different operation frequencies. (b) Upper panel: real-time moisture sensing with different RH ranges (all starting from 11% RH) and repeated RH detection between 11% RH and 90% RH for four cycles. Lower panel: response−recovery curve of the sensor with RH switching between 11 and 90%. (c) Complex impedance plots of the moisture sensor at varied RH levels. The real part and imaginary part are magnified simultaneously on the same plane for convenient comparison. Insets: corresponding equivalent circuits under low and high RH conditions. (d) Schematic illustration of the interactions (adsorption and penetration) between water molecules and GO nanosheets. (e) Long-term stability test of the moisture sensor. (f) Upper panel: impedance of the flexible sensor measured as a function of bending radius and bending strain, normalized to the initial value. Lower panel: changes of normalized impedance of the flexible sensor during cyclic bending test.

patterns (2 mm × 2 mm each) were manufactured on one large GO thin film (Supporting Information, Figure S2a), and their electrical properties were examined as a function of laser power (100, 110, and 120 mW) and writing speed (10, 30, 50, and 70 mm s−1), respectively. The electrical properties of the as-written rGO square patterns were tested by the four-probe measurement method (Supporting Information, Figure S2b). As presented in Figure 3a, the sheet resistance (Rs) of rGO squares produced with identical writing speeds drops as the laser power increases, which is because of a higher degree of photoreduction under stronger laser irradiations.25 At each laser power level, Rs decreases with the increase of the writing speed. This could be correlated with the change of the film thickness, which is determined by a competing effect between the oxidative ablation-induced mass loss25,27 and photothermal

expansion20,28−30 during the FsLDW. As plotted in Figure S3 (Supporting Information), a higher writing speed produces a thicker rGO film, resulting in a lower Rs. The parametric study demonstrates a tunable Rs by simply adjusting the FsLDW parameters, which prompts the fabrication of various devices based on the as-produced conducting materials. The rGO thin film obtained under the optimal condition registers an Rs value of around 1200 Ω sq−1. The photoreduction was first characterized using UV−vis spectroscopy on rGO written with optimized parameters. As shown in Figure 3b, the pristine GO thin film displays a strong absorption peak at 223 nm attributed to π−π* transition of CC bond and a shoulder at 298 nm owing to n−π* transitions of CO bonds. Upon laser irradiation, the π−π* absorption peak red shifts to 259 nm, whereas the n−π* D

DOI: 10.1021/acsami.7b13701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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lowest Rs; the final laser power and writing speed were 120 mW and 70 mm s−1, respectively. A customized humidity control system was developed so as to investigate the sensitivity and dynamic responses of the as-produced sensors (Supporting Information, Figure S5). To explore the sensitivity and response linearity of the device, the impedance changes with varied relative humidity (RH) levels were measured by applying a 1 V AC voltage at different frequencies (0.02, 0.1, 1, 10, and 100 kHz). As presented in Figure 4a, in the relatively lowfrequency region, the sensor exhibits a high sensitivity with the impedance changing from 56 MΩ at 15% RH to 90 kΩ at 95% RH. The impedance variation spans over three orders of magnitude (from 107 to 104 Ω), and the response is linear (in log scale) with respect to the RH in the range of 15−95%. The sensitivity is around 35 kΩ per % RH in the frequency range of 20 Hz to 1 kHz. The real-time sensing properties were measured at different RH levels at a fixed operation frequency of 1 kHz. Figure 4b reveals that the sensor shows high sensitivity at each RH value, wide dynamic range, and stable cyclic behavior. With the RH alternating between 11 and 90% at 23 °C, both the highest and the lowest impedance maintain their values almost invariant, indicating excellent reversibility and repeatability of the sensor. The response and recovery times of the sensor are estimated to be around 1.8 and 11.5 s, respectively (see Figure 4b); here, the response and recovery times were defined as the time spent to reach 90% of the total impedance change. Such a fast response−recovery behavior is comparable to or even outweighs that of precedent moisture sensors fabricated from sensing materials, such as VS2, GO, SnO 2 , and poly(ionic liquid)s, on metallic electrodes.11,16,17,38−40 The sensing mechanism was investigated in depth by the complex impedance spectra working at different RH levels (ranging from 11 to 95%). The complex impedances were measured over a frequency range of 20 Hz to 300 kHz with an AC voltage of 1 V at the room temperature of 23 °C. As shown in Figure 4c, the complex impedance spectrum at low RH (11%) manifests a half semicircle, whereas a semicircle is observed in the plot at 34% RH. Under low RH conditions, the physically adsorbed water molecules cannot move freely because of the hydrogen bond interaction with the GO surface (Figure 4d); hence, the intrinsic resistance and capacitance of the GO film attributed to the proton hopping among hydroxyl groups contribute to the sensor conductivity.17,38 The complex impedance curve can be modeled by an equivalent resistance− capacitance parallel circuit (Figure 4c, inset (I)). With the increase of RH (51, 63, 75, 87, and 95%), a larger amount of water molecules are physically adsorbed onto the GO nanosheets. The multilayer-adsorbed and penetrated water molecules are mobile so they can be ionized upon the applied electrostatic field, forming hydronium ions (H3O+) as the major charge carriers. As a result, ionic conduction dominates in the high RH environment, leading to the decreased impedance values. The complex impedance plot, therefore, shows a depressed semicircle (at high frequency) connecting a straight line (at low frequency). The corresponding equivalent circuit consists of a series combination of the GO film’s resistance and the impedance at the interface between GO film and rGO electrodes, in parallel with the capacitance that characterizes the influence of the polarization of the adsorbed water molecules on the GO film (Figure 4c, inset (II)).38−41 Therefore, the sensing mechanism could be attributed to proton and ionic conductivity in the low and high humidity conditions,

transition peak disappears, suggesting the deoxygenation of GO and the partial restoration of the conjugated structure.31 The removal of OCGs during the laser scanning on GO thin films was further investigated by Fourier-transform infrared (FTIR) spectroscopy. As shown in Figure 3c, the absorbance bands at 1730 and 1069 cm−1 in the FTIR spectrum of GO originate from CO and C−O stretching vibrations, respectively. The frequencies at 3620 and 1399 cm−1 correspond to O−H stretching and bending vibrations, respectively. Notably, these characteristic modes of OCGs are almost absent in the FTIR spectrum of rGO, indicating the effective elimination of OCGs upon laser irradiation. Furthermore, the peak of CC stretching mode appears at lower wavenumbers of rGO (1555 cm−1) compared with that of GO (1625 cm−1), which implies a restoration of the conjugated aromatic network.32 The X-ray diffraction (XRD) pattern collected from GO thin films shows an intense diffraction peak at 11.3° (Figure 3d), which corresponds to a (002) plane d-spacing of 0.78 nm. On the contrary, for rGO prepared by FsLDW, a weak and broad diffraction peak appears at 24.2°, indicating a decrease of the interlayer d-spacing down to 0.37 nm after photoreduction, owing to the effective removal of OCGs decorated on the basal planes.21 Raman spectroscopy has been intensively employed for the characterization of structural changes in graphitic materials.33 As displayed in Figure 3e, two dominant peaks observed from Raman spectrum of GO can be assigned to the D band at 1337 cm−1 and the G band at 1585 cm−1, respectively. After FsLDW of the pristine GO thin film, the D and G bands exhibit narrower features and downshifts in frequency to 1321 and 1568 cm−1, respectively. Because the G band is associated with the first-order Raman scattering of E2g mode of sp2 carbon atoms, whereas the D band arises from a second-order resonance process and is related to the disorder in the sp2 carbon structures,34 the overall changes in the D and G bands indicate a decrease in the structural defects and re-establishment of sp2 network after laser irradiation.35 Besides, the intensity ratio of D and G bands (ID/IG) drops from 2.08 for GO to 1.03 for rGO, implicating the increase in the average size of the sp2 domains owing to the removal of OCGs within the basal planes.18 Moreover, a downshifted (from 2689 to 2642 cm−1) and prominent 2D band and an S3 band (a Raman mode because of the combination of D and G bands) at 2901 cm−1 are observed from the Raman spectrum of rGO, implying better graphitization upon photoreduction of GO.36 In addition, X-ray photoelectron spectroscopy (XPS), a surface-sensitive technique that measures the elemental composition of a material, was further applied for quantitative evaluation of the photoreduction of the GO after laser irradiation. As shown in Figure 3f, the high-resolution C1s spectrum of GO could be deconvolved into four species with binding energies at approximately 284.6 (C−C/CC bond), 285.8 (C−OH bond), 286.8 (C−O−C bond), and 288.7 eV (CO bond), respectively.37 For rGO, on the other hand, the intensities of all oxygen-related C1s peaks decreased dramatically, pointing to the efficient deoxygenation and conversion of sp3-type carbon to sp2-type carbon in the network.35 The C/O atomic ratio change from 2.3 for GO to 6.9 for rGO obtained by the XPS survey scan (Supporting Information, Figure S4) also reveals the decrease of oxygen content as a result of laser reduction. The designed all-graphene in-plane sensor features the laserpatterned rGO domains as the electrodes; therefore, the optimal processing condition was selected for providing the E

DOI: 10.1021/acsami.7b13701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Long-range, selective, and high-spatial-resolution sensing capabilities. (a) Variation in RH of a fingertip (calibrated by a reference hygrometer) at different sensing distances. Scale bar: 20 mm. (b) Real-time responses of the sensor to human breath (green curve) and wind from portable fan (orange curve). Scale bar: 20 mm. (c) Distribution of ΔZ/Z0 obtained from each individual pixel when a pencil with wet end was placed at 1 mm above the e-skin matrix. Upper panel shows a sharp tip; bottom panel presents a thick tip.

substrate.43 These results indicate that our sensor is sufficiently reliable for practical e-skin applications. The noncontact sensing capabilities of the sensor were accessed by measuring the output signals induced by external moisture stimuli. As shown in Figure 5a, the RH distribution around a fingertip was precisely traced by gradually approaching the fingertip toward the sensor. The device started to show responses when the fingertip was located 10 mm away, demonstrating the successful mimicry of the long-range sensitivity of human skin. Another functionality of the human skin is providing quantitative feedback on the RH level in an open air environment; an example case study of the asfabricated sensor is shown in Figure 5b. The impedance level of the device is highly sensitive to the RH change induced by human breath (green curve in Figure 5b); on the contrary, there is no noticeable response to the wind blown by a portable fan (orange curve in Figure 5b). This selective sensitivity frees the sensor from other nonmoisture external disturbances when it is deployed as the noncontact e-skin. When an e-skin is utilized for noncontact sensing applications, it is of significance not only to observe responses toward the stimulus but also to detect the position of the stimulus. In this regards, an in-plane 4 × 4 sensing matrix was directly fabricated on a flexible PET substrate by virtue of the scalable writing capability of FsLDW, as shown in Figure S8 (Supporting Information). The noncontact sensing properties of the e-skin matrix in 3D space were subsequently explored. In analogy with the results shown in Figure 5a, each individual pixel in the matrix exhibits a long-range and depth-resolved (along Z direction) moisture-sensing capability. Its lateral (in X−Y plane) sensing resolution was assessed by applying moisture stimuli from sources with two different geometries. A pencil with wet ends was utilized for this purpose as shown in Figure 5c. When the sharp tip was positioned at 1 mm above

respectively. On the basis of this sensing mechanism, the moisture-sensing performance of the device was also investigated at an elevated temperature of 50 °C at a fixed frequency of 1 kHz. As plotted in Figure S6 (Supporting Information), the impedance of the device decreases almost linearly (in log scale) with the increase of RH at 50 °C, whereas the impedance changing range is narrower compared to that at 23 °C. This can be explained that the intrinsically trapped water molecules in GO nanosheets are fewer when the temperature is elevated (50 °C), generating fewer ions when the RH increases, resulting in a narrower impedance changing range. High stability and durability against the long-term exposure to environmental disturbances and mechanical deformations are the prerequisites for the flexible sensors used in our daily lives. As plotted in Figure 4e, the measured impedance operating at 1 kHz shows nonobvious fluctuation (less than 0.1%) at each RH level for 30 days, confirming the long-term stability of the sensor. The mechanical flexibility of the sensor was examined under bending deformation (Supporting Information, Figure S7) at ambient conditions (23 °C and 46% RH). The normalized impedance (Z/Z0) was measured as a function of bending radius (R) and plotted in the upper panel of Figure 4f. The corresponding bending strains (ε) derived from ε = h/2R (where h = 100 μm is the thickness of the PET substrate)42 are also indicated in the graph. The sensor exhibits a neglect impedance change (Z/Z0 < 1.05) even when it is subject to a severe bending deformation with a bending strain of 1.32%. Furthermore, as presented in the lower panel of Figure 4f, even after 1000 bending cycles with a bending radius of 8 mm, the impedance almost sustains its original value (Z/Z0 < 1.07), confirming the high durability of the flexible sensor. Such an excellent mechanical robustness could be attributed to the high specific surface area of the flat GO/rGO thin films that permits their strong interfacial interaction with the PET F

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onto the surface of the GO thin films that were placed on an X−Y translation stage. The sensors were fabricated by implementing the programmable FsLDW technique. Upon the deposition of GO thin film on the PET substrate, an rGO electrode with 10 interdigitated fingers was directly written in the geometry having a width of 300 μm, spacing of 300 μm, and length of 8 mm by raster scanning the infrared laser beam. Two square pads were created as two terminals of the sensor for electrical wiring. Similarly, a 4 × 4 sensing matrix was also demonstrated by single-step FsLDW on a PET substrate. As schematically illustrated in Figure 5c, the pattern size of each individual pixel is 5 mm × 5 mm, and the interpixel spacing is 5 mm. Characterizations. Optical images of the as-written patterns were captured using a stereomicroscope equipped with a charge-coupled device camera (AmScope). The morphologies of the GO and rGO thin films were characterized with a field-emission SEM (JEOL, JSM7600F). The UV−vis spectra were collected from GO and rGO thin films deposited on quartz coverslips (Alfa Aesar) with a UV-2450 spectrometer (Shimadzu Corp.). The infrared transmission spectra were recorded on an FTIR spectrometer (Thermo Scientific, Nicolet 6700) by dispersing the samples in KBr pellets. The XRD patterns were obtained with a PANalytical Empyrean diffractometer equipped with a Cu Kα (λ = 1.5406 Å) X-ray source. Raman spectra were acquired using a confocal Raman system (Renishaw, inVia Raman microscope) with a 633 nm (HeNe, Renishaw, RL 633) excitation source. The laser power was kept below 0.5 mW on samples with a spot size of 1 μm. XPS was performed using a PHI-5400 spectrometer with Al Kα (1486.6 eV) radiation. The binding energies were calibrated with reference to CC species located at 284.6 eV. The thicknesses of GO and rGO thin films were measured using a Dektak XT surface profiler. Electrical measurements were carried out on 2 mm × 2 mm rGO squares shown in Figure S2a (Supporting Information) using a fourprobe setup. Four probes (made of tungsten tip) were placed at the corners of the square pattern, as illustrated in Figure S2b (Supporting Information). Current−voltage (I−V) characteristics were measured using a source-measurement unit (Keysight, B2902A) by applying a linear current between probe 1 and 2 (I1,2) and recording the voltage drop between probe 3 and 4 (ΔV3,4). The Rs of rGO squares could be derived based on a van der Pauw model using eq 119

one pixel among the matrix, it only resulted in a significant change in impedance to the pixel itself, whereas the responses were negligible at nearby pixels (refer to the 3D plot in the upper panel). On the contrary, when the matrix was facing the thick tip of the pencil (also with a 1 mm separation distance), together with the prominent impedance change observed at the most proximal pixel, other neighboring pixels were affected as well (refer to the 3D plot in lower panel). These results demonstrate the high-spatial-resolution sensing capability of the e-skin matrix, rendering it the potential in prosthetics. This noncontact e-skin can be facilely implemented by parallel electrical connection with a portable impedance converter and a microcontroller for time-multiplexed measurements of 16 individual cells in the matrix.44

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CONCLUSIONS In conclusion, we demonstrated a straightforward strategy to fabricate all-graphene, flexible, and noncontact e-skins exploiting the moisture-sensing mechanism. A programmable FsLDW technique, which allows for designable patterning and controllable photoreduction of GO thin films, was utilized to directly create all-graphene devices comprising rGO as the electrodes and GO as the sensing material. The as-produced sensors exhibited high sensitivity, fast response, rapid recovery, good long-term stability, and excellent mechanical robustness. The FsLDW also enabled scalable sensor integration on flexible substrates in a single step, which surpasses the conventional cumbersome and complex manufacturing processes. A 4 × 4 sensing matrix was developed for prototype demonstration of a noncontact e-skin with mechanical robustness, showing excellent long-range, moisture selective, and high-spatialresolution sensing performances. Our work will pave the way for the development of green and deformable noncontact eskins.

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METHODS

Preparation of GO Thin Films. GO nanosheets were synthesized from natural graphite powder (Fluka 50870) using the modified Hummers method according to our previous study.45 The as-prepared GO nanosheets (Supporting Information, Figure S9) were then homogeneously dispersed in deionized (DI) water with the assistance of ultrasonication. Melinex PET films (Dupont Teijin Films), which served as the highly flexible substrates, were cleaned by ethanol and then blow-dried with N2 gas. To enhance the interfacial interaction between GO and PET substrates, thus increasing the mechanical robustness of the as-fabricated devices, the well-cleaned PET films were treated by O2 plasma at 18 W for 1 min prior to the deposition of GO aqueous suspension (1 mg mL−1) using a micropipette. Finally, the specimen was heated on a hotplate at 50 °C for 30 min to obtain a uniform GO thin film. Fabrication of Noncontact Sensors via Programmable FsLDW. A standard platform was established for carrying out FsLDW on the as-prepared GO thin films, as illustrated in Figure 2a. A femtosecond Er-doped fiber laser (Toptica, FemtoFiber pro NIR) with second-harmonic wavelength at 780 nm, pulse duration of 100 fs, and a repetition rate of 83 MHz was utilized as the laser beam source. A half-wave plate (HWP) and a polarizing beam splitter (PBS) were employed to accurately adjust the laser power. Another beam splitter (BS) was placed afterward for measuring the laser power using a power meter (PM). The laser beam was directed by dichroic mirrors (M) and a two galvano mirror (GM) system, and finally focused on a two-dimensional plane through an f-theta lens (focal length, f = 100 mm). The focused laser spot was scanned laterally by steering the GMs, with a scanning speed up to 1 m s−1. According to the predesigned programs, arbitrary patterns could be directly printed

Rs =

π ln

ΔV3,4 2 I1,2

(1)

Evaluation of Sensors. The sensitivity, response linearity, and sensing mechanism of the as-fabricated sensors toward moisture were probed by recording the impedance change at different frequencies with an LCR meter (Keysight, E4980AL) against RH levels from 11 to 95% inside a customized humidity controlling chamber at the constant temperature of 23 °C. As sketched in Figure S5a (Supporting Information), the RH level was adjusted by inletting water vapor (bubbled by N2 gas) to the chamber and monitored with a reference hygrometer (Omega RH32). The dynamic responses of the sensors were characterized by monitoring the impedance change at a fixed operation frequency of 1 kHz while rapidly alternating the RH in the test chamber between two levels. A fast-flowing N2 stream was introduced into either a sulfuric acid container (low RH) or a DI water container (high RH) using a three-way valve (Figure S5b and Video S1 in the Supporting Information). The RH values could be standardized according to the data obtained from the static measurements. The flexibility test was carried out by fixing one end of the sensor and moving the other end through a translation stage. As shown in Figure S7 (Supporting Information), the bending radius and the bending strain of the sensor can be tuned by adjusting the distance between the two holders. Repetitive bending test was conducted using a caliper with the bending radius of 8 mm. All mechanical measurements and noncontact proximity sensing tests were performed at a fixed operation frequency of 1 kHz under ambient conditions (temperature of 23 °C and RH of 46%). G

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13701. Arbitrary patterning on GO thin films; sheet resistance measurements on as-written rGO thin films; thickness of rGO thin films produced with different writing speeds; broad scan XPS spectra of GO and rGO thin films; schematic configuration of the humidity controlling system; bending test of the flexible device; photographs of an e-skin conformably attached on a human hand; and atomic force microscopy image of the as-prepared GO nanosheets (PDF) Fast-flowing N2 stream introduced into either a sulfuric acid container or a DI water container (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.S.). *E-mail: [email protected] (Y.-J.K.). ORCID

Zhaoyao Zhan: 0000-0003-4946-3842 Wei Huang: 0000-0001-7004-6408 Gengzhi Sun: 0000-0002-8000-8912 Young-Jin Kim: 0000-0002-4271-5771 Author Contributions

J.A., G.S., and Y.-J.K. conceived the idea and designed the experiments. J.A. and T.-S.D.L. performed sample fabrication. J.A. and Y.H. carried out moisture-sensing experiments. J.A., T.S.D.L., Z.Z., and Y.L. conducted characterizations. J.A., L.Z., W.H., G.S., and Y.-J.K. prepared the manuscript. All authors contributed significant discussions for final paper polishing. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Singapore National Research Foundation (NRF-NRFF2015-02) and Tier 1 Grant (RG180/ 16) from the Singapore Ministry of Education.

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DOI: 10.1021/acsami.7b13701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX