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Highly Conductive, Flexible, and Compressible All-Graphene Passive Electronic Skin for Sensing Human Touch Chengyi Hou, Hongzhi Wang,* Qinghong Zhang, Yaogang Li,* and Meifang Zhu
Skin, the largest organ of the body, permits the sensations of pressure, temperature and pain, and is therefore an important source of crucial information. Skin is used not only to receive information about objects, but also for social interaction by allowing the communication of feelings and emotions through body contact (e.g., hand, leg, or torso). Body contact, in particular skin-to-skin contact, induces both a temperature variation and pressure on the skin, and thus can offer strong emotional and memory ties.[1–3] Nowadays, it has been widely accepted that flexible thin-film sensors (FTSs), which mimic skin sensitivity, will be very important in the future for biomedical (or prosthetic) and robotic applications.[4–7] This is especially true of multiple pressure/temperature FTSs that not only convey the complex types of information given by body contact, but also can discern the human body from other objects, and may introduce the emotional communication capability required for next-generation robots. Very recently, a great deal of attention has been placed on pressure sensing FTSs that exhibit mechanical properties comparable to human skin, and that sense mechanical stress by measuring the total integrated signal[8–10] or resolve texture at a certain resolution.[4–6,11,12] These devices have successfully mimicked the pressure sensing abilities of human skin, but have largely ignored other crucial functions those help us respond properly to surroundings or people, such as temperature sensitivity. Though the sensing behaviors of the abovementioned pressure sensors (or electronic skin) are inherently temperature-related, these FTSs are actually incapable of detecting temperature or locating the heated/cooled area. Instead, their response must be calibrated with an additional temperature sensor. To the best of our knowledge, there have been few efforts concentrating on a skin-like flexible thin-film
Dr. C. Hou, Prof. H. Wang, Prof. Q. Zhang, Prof. M. Zhu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials College of Materials Science and Engineering Donghua University Shanghai 201620, PR China E-mail: [email protected] Prof. Y. Li Engineering Research Center of Advanced Glasses Manufacturing Technology (MOE) College of Materials Science and Engineering Donghua University Shanghai 201620, PR China E-mail: [email protected]
DOI: 10.1002/adma.201401367
Adv. Mater. 2014, DOI: 10.1002/adma.201401367
temperature sensor[13–15] or multiple sensor that is able to sense pressure and temperature simultaneously.[16] Additionally, the current methods used for the fabrication of multiple FTSs and other individual pressure and temperature sensing FTSs require either large-scale integration of high-performance electronic component (e.g., field-effect transistors) on flexible substrates or the homogeneous compositing of functional materials with polymer matrices. These and other related technologies have value in a certain context, but they involve special processing steps and complex fabrication designs that do not offer facile methods for widespread practical applications of FTSs. Moreover, current FTSs require either an external or an integrated internal power supply (or working voltage) to demonstrate sensitivity,[4–6,10–13,16,17] which is very different from the behavior of human skin and will limit their use in many applications. Easy, fast, and cost-effective fabrication techniques for passive multiple FTSs are still in high demand. To meet functional and practical requirements, this work focuses on developing a new type of passive multiple FTS based upon only one functional material: graphene. Considering its excellent mechanical, electrical, and thermoelectric properties, it is not surprising that graphene nanosheets have been increasingly integrated into macroscopic materials for a wide range of real-world applications including graphene papers,[18–21] graphene foams,[22,23] graphene fibers[24–27] and graphene gels.[28–32] Many varieties of macroscopic graphene materials can be prepared from graphene oxide (GO) precursors, which is a lowcost strategy.[33] Moreover, we have previously demonstrated that graphene foams hold promise for pressure sensing FTSs because of their compressive porous structures.[7] Inspired by these prospects, the work produces a novel reduced graphene oxide foam (RGOF) that is free-standing, flexible, and elastic. Owing to its outstanding electrical properties, this RGOF demonstrates a temperature sensitivity based upon thermoelectric effects in the graphene. Furthermore, this RGOF also demonstrates pressure sensing behaviors under finger pressure based upon finger heating effects. Finally, this work also produces a proof-of-concept RGOF pressure sensor pad that can locate finger-pressure points and measure the pressure levels. Most importantly, all of the sensing abilities of this device are demonstrated without any internal/external power supply. Figure 1a and 1b show the cross-sectional field-emission scanning electron microscope (FESEM) images of the RGOFs, where it can be seen that these paper-like foams have open porous networks. These samples are prepared by vacuum filtering a frozen GO solution, followed by hydriodic acid (HI) reduction, washing and freeze drying. The porous struc-
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nanostructure and the electrical behaviors of RGOFs can be easily tuned by human touch (ca. 1–10 kPa,[4,7,36] which contributes to a high sensitivity to pressure loads. This study examines the thermoelectric behaviors of the RGOF through potentiostatic testing at a working voltage of 0 V. As shown in Figure 2a, the foam is integrated with small contact electrodes (platinum films) on the upper and lower surfaces, and then placed on a flat substrate (maintained at ca. 20 °C) while its upper layers are partially heated at different temperatures. We detect a remarkable temperature difference (ΔT) between the heated surface (x) and other areas of the device (y and z). Consequently, positive currents are detected by electrodes on either side of the top surface (x-y) and on the top and bottom (x-z) (Figure 2b). Additionally, the current curve is consistent with the surface temperature curve as a function of time (Figure 2c), which demonFigure 1. The morphologies and multicycle compressive properties of the RGOF. a,b) Cross- strates a precise and continuous thermal sectional FESEM images of the RGOF. c) Photographs of the free-standing and flexible paper- characterization ability. The above results like foam. d) Electrical resistance change when repeatedly compressed for over 19 cycles. The also indicate that ΔT guides the direction of WE and CE/RE stand for working electrode and counter/reference electrodes, respectively, and the temperature-activated carrier transport, F indicates the compression force. as illustrated in Figure 2a. The thermocurrent is around 9 µA for the RGOF sample possessing a 5.6 cm2 heating surface at a ΔT of about 30 °C, ture of RGOFs is a result of the freeze-casting induced in the above process. In freeze-casting, porous networks are formed which is measured with electrodes x and z. This value is only in frozen GO solution due to the ice template[28] and the netslightly lower than that measured with electrodes x and y (ca. 10 µA), which indicates the good carrier transmission capability work retains its connectivity when the ice is thawed and filof the RGOF in the thickness direction, which can be attribtered (Figure S1, Supporting Information). The RGOFs posuted to the graphene networks built during the freezing and sessing this nanostructure are free-standing and highly flexible filtering processes (Figure 1b). Additionally, the thermocurrent (Figure 1c), and more importantly, exhibit a sheet resistance as exhibits high stability and reversibility with successive on-off low as 1.5 ± 0.3 Ω sq−1, which is almost two orders of magheating (Figure S8, Supporting Information). The output of nitude lower than the lowest values ever reported from an the RGOF with a reverse connection to the external circuit is RGOF (ca. 100 Ω sq−1,[22] 91.2 Ω sq−1.[23] Additionally, the conalso measured, and it is found that the typical thermocurrent ductivity of these RGOFs (53 ± 3 S cm−1) is even better than remains roughly the same (ca. 9 µA) but the polarity is reversed that of the chemical vapor deposition-derived graphene foams (Figure S9, Supporting Information). This result confirms that (ca. 10 S cm−1).[23] This remarkably enhanced electrical property the measured electric signals are generated by the RGOF and may be attributed to highly conductive contacts between the not the measurement system. Figure 2d and S10 show the thergraphene sheets in our RGOF, as well as HI reduction-induced moelectric current as a function of ΔT. To measure the thermoiodine doping (Figure S2, S3, and Table S1, Supporting Inforelectric behaviors at ΔT < 0, the whole device is initially mainmation).[34,35] These results give the assurance of a good thertained at various temperatures and its surface is subsequently moelectric performance from these RGOF foams. cooled using ice. It is found that the thermoelectric current Remarkably, the RGOF can be compressed to at least 65% increases remarkably with increasing |ΔT| and, for a 5.6 cm2 strain and is still able to recover most of its original thickness rapidly when the loading is removed (see Figure S4 and heating surface, the current can reach around 550 µA at a ΔT of Movie S1, Supporting Information). Detailed hysteresis studies about 97.0 °C. Moreover, we find that the current increases lin(Figure S5, Supporting Information) show that it recovers at early when the heating surface is either in a free or a bending least at the speed of >180 mm min−1. Additionally, as shown state (Figure 2e). These results show that our RGOF is capable of sensing heat and cold, and of measuring the heated/cooled in Figure 1d and S6, the electrical resistance of the foam area under zero working voltage. decreases by 11% when the foam is compressed up to 68% strain under 2 kPa of pressure. The electrical properties can More interestingly, we find that the RGOF is capable of rapidly and completely recover to the original value within ca. discerning the human body from other objects (e.g., plastic, 0.05 s (Figure S7, Supporting Information), and the response metal, glass, rubber, etc.) under ambient conditions. As finger of the electrical resistance is found to be highly constant over pressure is applied, the output characteristics of the RGOF multiple cycles of compression. These results indicate that the undertake pronounced changes; while the read-out current
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COMMUNICATION Figure 2. Temperature sensing behaviors of the RGOF. a) Schematic of the device geometry illustrating the temperature-activated carrier transport through the temperature difference direction. b) Time-dependent thermocurrent measurements of the RGOF under on–off heat irradiation, where irradiation lasts 30 s (pink area). The thermocurrent is measured by electrodes x-y (red curve) and x-z (violet curve). c) Time-dependent thermocurrent and surface temperature curves of the RGOF under on-off heat irradiation. d) The ΔT-dependent thermocurrent curve of the sensor. The inset shows the magnified thermocurrent curve under the low ΔT range. e) Heating surface-dependent thermocurrent measurements of the sensor, which has been bent at different curvatures (r), as illustrated in the inset. f) Photographs show the sensor being touched by a human finger and other common objects. The current is measured by electrodes x-y (as illustrated in Figure 2a). Scale bar: 15 mm. The thermocurrent is measured by electrodes x-z in (c–e). The error bars are the the standard deviations calculated from repeated measurements.
remains unchanged when the RGOF is pressed using other common objects (Figure 2f and Movie S2–S4, Supporting Information). This is because body temperature is generally much higher than the temperature of our surroundings (Figure S11, Supporting Information), indicating that surface temperature is the most significant difference between a human body and an inanimate object. Therefore, owing to the unique temperatureactivated sensing mechanism, our RGOF sensor can recognize human touch. This unique feature can be the cornerstone for achieving the capability of social interaction or emotional communication required in next-generation robots.
Adv. Mater. 2014, DOI: 10.1002/adma.201401367
Encouraged by the efficient temperature-activated sensing behaviors of the RGOF, we subsequently demonstrate that the RGOF is able to sense human touch/pressure based on fingerheating effects. Figure 3a displays an infrared thermal image of the RGOF sensor. A finger touch on the sensor of no more than 1 s is able to generates a change in the maximum surface temperature, which can be seen in Figure 3a (area I; ΔT = 4.1 °C) where arrows indicate the carrier current due to temperature differences. Electric signals were thus detected by electrodes. Figure 3b and 3c show the electrical output performance of
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Figure 3. Characterization of the human touch response of the RGOF sensor. a) Infrared thermal image of the sensor surface, where area I is touched for 0.53 s, leading to a notable increase in surface temperature. Arrows indicate the carrier current. b) The current change upon placing and removing a finger (ca. 61 mm2) on the RGOF surface (areas I, II, and III), corresponding to a pressure of only 163 ± 12 Pa. c) The current change detected with a rapid and slow finger touch/remove cycle on area I, which reveals the high-speed and stable responsiveness of the RGOF sensor. Scale bar: 5 mm. d) Photograph (top panel) and corresponding infrared thermal image (middle panel) of the as-fabricated sensor pad. The bottom panel shows the bent sensor pad directly attached to a hand. Scale bar: 10 mm. e) Real-time current curves of the sensor pad during a finger touch/remove cycle on its surface. Curves shown in compartments A–I correspond to the touched areas A–I (see Figure 3d), respectively. Curves in each panel are measured by electrodes x1-y1 and x2-y2, as indicated in (E). Timescale bars: 5 s.
the RGOF sensor induced by finger touches. Pressures are measured to be 163 ± 12 Pa, which are very gentle touches that will not compress the foam. These finger-heating effects can have a direct impact upon the electrical behavior of RGOFs. The electrical signal displays high reversibility and stability and, more interestingly, according to Figure 3b, when the three different surface areas (I, II, and III) are touched the output current is about +200, 0, and –200 nA,
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respectively, which indicates that the RGOF sensors have the potential to determine the location of a finger touch by providing different types of electrical readout: positive, negative, or zero. We produce a proof-of-concept RGOF pressure sensor pad that is flexible and, except for the metal current collectors, an all-graphene device (Figure 3d). The device is fabricated by connecting an RGOF to four independent Cu foils
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COMMUNICATION Figure 4. Pressure sensing behaviors of the RGOF sensor. a) Infrared thermal images, real-time mean surface temperature curves and surfacetemperature distributions (corresponding to AR01 areas in infrared thermal images) of the RGOF (area A), demonstrating the thermal distributions on the surface of the sensor pad undergoing an ultra-light touch (165 Pa) or heavy pressure (49.0 kPa). Scale bars: 6 mm. b) Cross-sectional infrared thermal imaging macro photos, corresponding deep-dependent ΔT curves, and output current (measured by electrodes x2-y2) of the RGOF (area A), demonstrating the thermal distributions on the cross section of the sensor pad undergoing an ultra-light touch (165 Pa) or heavy pressure (49.0 kPa). The sensor pad in these measurements is placed on a Chinese coin (Figure S13, Supporting Information). The ΔT presents temperature changes in the thickness direction at the edge of the touched area. Insets show cross-sectional FESEM images of the RGOF under 165 Pa or 49.0 kPa. Scale bars: 2.5 µm. (c) Finger pressure-resolved current diagram on area A of the RGOF sensor-pad measured with electrodes x2-y2.
(x1, y1, x2, and y2) at its four sides whose metal collectors are connected to WEs and CEs/REs. This device can work as a 3 × 3 pixel pressure sensor pad that is capable of locating a fingertip touch. As illustrated in Figure 3d (middle panel) and 3e, the pressure sensor pad is partitioned into nine compartments, each using a ca. 64 mm2 area of the RGOF. This area limitation is due to the contact area of a typical fingertip (ca. 61 mm2, C. Hou’s index finger), and the current curves exhibited by each compartment displays the response of the device to a gentle touch (see Figure S12, Supporting Information). Based on the finger-heating effects, two ΔT-related signals are recorded as one result by the electrode lines x1-y1 and x2-y2 for each measurement. The device can give a total of nine different readout results that correspond to all possible locations in the 3 × 3 grid, thereby mapping the applied finger pressures with the sensor pad.
Adv. Mater. 2014, DOI: 10.1002/adma.201401367
An additional benefit of the elastic structure of the RGOF sensor pad is the ability to “map” the pressure levels by outputting the value-changeable thermocurrent. Typically, ultralight touches (163 ± 12 Pa) and heavy pressures (48.9 ± 0.3 kPa) are applied on the device. The temperature distribution on the surface (Figure 4a) and cross section (Figure 4b) of the device, together with the quantitative measurements, clearly show the diffusive characteristics of heat from the finger, which influences the thermocurrent. Figure 4a shows the obvious increase in the heating area (up to 99%) and decrease in the thermal diffusion velocity after finger removal when the RGOF is compressed under a relatively large pressure. Meanwhile, the rate of increase in the mean temperature difference also reaches 57% (from 2.4 to 3.3 °C), though it is worth noting that the finger contact area only increases by 3% in comparison. This indicates
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that the finger pressure rather than its contact area influences the finger heating effect to the greatest extent, wherein greater pressure by the finger induces a faster diffusion of the finger heat. The cross-sectional thermal imaging macro photos shown in Figure 4b further demonstrate the pressure-related thermal diffusion in the RGOF sensor pad in the thickness direction. Under relatively high pressure conditions, densification of the graphene networks is observed (FESEM images in Figure 4b), inducing a deeper diffusion of the finger heat due to the compact structure (Figure 4b). These results clearly show that a larger pressure induces more thermal energy in the RGOF. Finally, owing to the increasing thermal energy content, the readout thermocurrent measured from the compressed RGOF sensor pad increases to around 1500 nA, which is more than 7 times the value measured with a slight touch (ca. 200 nA). Figure 4c shows the finger pressure-resolved current diagram on area A of the RGOF sensor pad measured with electrodes x2-y2. The results indicate that the change in the finger pressure applied on the sensor-pad can be visually read from the output electrical signals. In addition to the fact that the all-graphene FTS in this study demonstrates desirable human touch-activated electrical behaviors, it also has a high sensitivity to pressure (p). The pressure from an ultra-light touch (163 ± 12 Pa) which does not initially compress the foam is defined as the base pressure for the devices, and any additional pressure is defined as the applied pressure (Δp). The sensitivity (S) can be defined as the slope of the trace in Figure S14 in the Supporting Information given by the relationship S = δ(ΔI/I0)/δΔp = (1/I0)δI/δΔp, where I is the current induced by applied pressure on the device and I0 is the device current with only a base pressure. The sensitivity of this device is found to be 15.2 kPa−1 when Δp is less than 300 Pa, which is much higher than the previously reported values obtained by an OFET sensor (8.4 kPa−1),[37] ferroelectret transistor sensor (10−3 kPa−1),[38] piezoelectric sensor (0.13 kPa−1),[39] capacitive sensor (0.55 kPa−1),[40] and resistance sensor (1.8 kPa−1).[17] This high pressure sensitivity relies mainly upon the high conductivity and the compressive (elastic) structure of this RGOF, and to the best of our knowledge, it has not been observed in other all-graphene thin-film materials. Recently, Park et al.[16] demonstrated a sensor array for the simultaneous sensing of pressure and temperature under an applied alternative current voltage with an amplitude of 5 V and a frequency of 0.3125 Hz. By comparison, our device achieves high sensitivity without applying any working voltage, which is one step closer to natural skin attributes. An unpowered FTS such as the one in this report is a passive device that relies solely upon the energy transmitted from the stimulus source, which is naturally available, and it is therefore more environmentally friendly than a self-powered sensor that uses its own electrical power from sources such as batteries, piezoelectric generator and solar cells. In conclusion, this report successfully demonstrates a highly conductive, flexible, and compressible all-graphene thin-film sensor that can sense heat and cold, measure the dimensions of the heated/cooled area, discern human touch from other pressures, and enable human touch locating and pressure level measuring under zero working voltage. The unique passive
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all-graphene FTS reported here may open up new doors for the development of electronic skin.
Experimental Section Synthesis of RGOFs: All the reagents were of analytical grade and used as obtained without further purification. The GO was synthesized through the Hummers’ method.[41] The GO foams were prepared by vacuum filtering a frozen GO solution using an Anodisc membrane (pore diameter: 0.2 µm), which were subsequently peeled off of the membrane and immersed into a 55% HI solution at room temperature for 1 h. The HI-reduced RGOFs were washed with deionized water several times. Fabrication of RGOF Sensor-Pad: The device was fabricated by connecting a RGOF to four independent Cu foils (x1, y1, x2, and y2) at its four sides. These metal collectors x1 and x2 were connected to CE/REs while y1 and y2 were connected to the WEs of the electrochemical workstations (CHI760D, Shanghai Chenhua Instruments) in measurements. Characterization and Measurements: The morphologies of the as-prepared samples were determined using a JSM-6700F FESEM, and the photographs were taken using a charge-coupled device (CCD) video camera (PowerShot G10, Canon). The electrical resistance of the as-prepared products was measured by the two-probe method using a Zahner electrochemical workstation (Zennium CIMPS-1), the applied voltage was 5 V. Compression experiments were conducted with a digital vernier caliper and an universal testing machine (INSTRON 5969). The electrical conductivity and sheet resistance were measured using the 4-point probe method (MCP-360, Mitsubishi Chemical Analytech Co. Ltd.) at room temperature. The thermoresponsivity measurements were carried out on an electrochemical workstation (CHI760D, Shanghai Chenhua Instruments). The RGOFs were integrated with small contact electrodes (Pt or Cu films) on the upper and lower surfaces, whose electrodes were connected to an electrochemical workstation. A solar simulator (Newport) equipped with a 300 W xenon lamp was used as a thermal source, whose light intensity was adjusted using an National Renewable Energy Laboratory-calibrated Si solar cell with a KG-1 filter for approximating one sun light intensity with Air Mass 1.5 Globlal (AM 1.5G). Ice and an electric heater were also used to control the temperature of the sensor’ surfaces. The integrated RGOF sensor pad was pressed by human fingers. The output current was measured under 0 V. The temperatures and infrared thermal images were recorded using an FLIR Thermo-Vision A40M infrared thermometer. The finger pressure (p) was calculated using the relationship p = N/S, where N is the load and, S is the area of the finger touch. A Mettler Toledo AL204 laboratory balance was placed under the device to collect the N data, and S was calculated by the grid method.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors gratefully acknowledge the financial support by the Natural Science Foundation of China (No. 51172042), the HighTech Research and Development Program of China (2012AA030309), the Specialized Research Fund for the Doctoral Program of Higher Education (20110075130001), the Science and Technology Commission of Shanghai Municipality (12nm0503900, 13JC1400200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Program for Changjiang Scholars and Innovation Research Team in University (TR2011079, IRT1221), the Fundamental Research Funds for the Central Universities, and
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Adv. Mater. 2014, DOI: 10.1002/adma.201401367
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Received: March 27, 2014 Revised: April 22, 2014 Published online:
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Highly Conductive, Flexible, and Compressible All-Graphene Passive Electronic Skin for Sensing Human Touch Chengyi Hou, Hongzhi Wang,* Qinghong Zhang, Yaogang Li,* and Meifang Zhu
Skin, the largest organ of the body, permits the sensations of pressure, temperature and pain, and is therefore an important source of crucial information. Skin is used not only to receive information about objects, but also for social interaction by allowing the communication of feelings and emotions through body contact (e.g., hand, leg, or torso). Body contact, in particular skin-to-skin contact, induces both a temperature variation and pressure on the skin, and thus can offer strong emotional and memory ties.[1–3] Nowadays, it has been widely accepted that flexible thin-film sensors (FTSs), which mimic skin sensitivity, will be very important in the future for biomedical (or prosthetic) and robotic applications.[4–7] This is especially true of multiple pressure/temperature FTSs that not only convey the complex types of information given by body contact, but also can discern the human body from other objects, and may introduce the emotional communication capability required for next-generation robots. Very recently, a great deal of attention has been placed on pressure sensing FTSs that exhibit mechanical properties comparable to human skin, and that sense mechanical stress by measuring the total integrated signal[8–10] or resolve texture at a certain resolution.[4–6,11,12] These devices have successfully mimicked the pressure sensing abilities of human skin, but have largely ignored other crucial functions those help us respond properly to surroundings or people, such as temperature sensitivity. Though the sensing behaviors of the abovementioned pressure sensors (or electronic skin) are inherently temperature-related, these FTSs are actually incapable of detecting temperature or locating the heated/cooled area. Instead, their response must be calibrated with an additional temperature sensor. To the best of our knowledge, there have been few efforts concentrating on a skin-like flexible thin-film
Dr. C. Hou, Prof. H. Wang, Prof. Q. Zhang, Prof. M. Zhu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials College of Materials Science and Engineering Donghua University Shanghai 201620, PR China E-mail: [email protected] Prof. Y. Li Engineering Research Center of Advanced Glasses Manufacturing Technology (MOE) College of Materials Science and Engineering Donghua University Shanghai 201620, PR China E-mail: [email protected]
DOI: 10.1002/adma.201401367
Adv. Mater. 2014, DOI: 10.1002/adma.201401367
temperature sensor[13–15] or multiple sensor that is able to sense pressure and temperature simultaneously.[16] Additionally, the current methods used for the fabrication of multiple FTSs and other individual pressure and temperature sensing FTSs require either large-scale integration of high-performance electronic component (e.g., field-effect transistors) on flexible substrates or the homogeneous compositing of functional materials with polymer matrices. These and other related technologies have value in a certain context, but they involve special processing steps and complex fabrication designs that do not offer facile methods for widespread practical applications of FTSs. Moreover, current FTSs require either an external or an integrated internal power supply (or working voltage) to demonstrate sensitivity,[4–6,10–13,16,17] which is very different from the behavior of human skin and will limit their use in many applications. Easy, fast, and cost-effective fabrication techniques for passive multiple FTSs are still in high demand. To meet functional and practical requirements, this work focuses on developing a new type of passive multiple FTS based upon only one functional material: graphene. Considering its excellent mechanical, electrical, and thermoelectric properties, it is not surprising that graphene nanosheets have been increasingly integrated into macroscopic materials for a wide range of real-world applications including graphene papers,[18–21] graphene foams,[22,23] graphene fibers[24–27] and graphene gels.[28–32] Many varieties of macroscopic graphene materials can be prepared from graphene oxide (GO) precursors, which is a lowcost strategy.[33] Moreover, we have previously demonstrated that graphene foams hold promise for pressure sensing FTSs because of their compressive porous structures.[7] Inspired by these prospects, the work produces a novel reduced graphene oxide foam (RGOF) that is free-standing, flexible, and elastic. Owing to its outstanding electrical properties, this RGOF demonstrates a temperature sensitivity based upon thermoelectric effects in the graphene. Furthermore, this RGOF also demonstrates pressure sensing behaviors under finger pressure based upon finger heating effects. Finally, this work also produces a proof-of-concept RGOF pressure sensor pad that can locate finger-pressure points and measure the pressure levels. Most importantly, all of the sensing abilities of this device are demonstrated without any internal/external power supply. Figure 1a and 1b show the cross-sectional field-emission scanning electron microscope (FESEM) images of the RGOFs, where it can be seen that these paper-like foams have open porous networks. These samples are prepared by vacuum filtering a frozen GO solution, followed by hydriodic acid (HI) reduction, washing and freeze drying. The porous struc-
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nanostructure and the electrical behaviors of RGOFs can be easily tuned by human touch (ca. 1–10 kPa,[4,7,36] which contributes to a high sensitivity to pressure loads. This study examines the thermoelectric behaviors of the RGOF through potentiostatic testing at a working voltage of 0 V. As shown in Figure 2a, the foam is integrated with small contact electrodes (platinum films) on the upper and lower surfaces, and then placed on a flat substrate (maintained at ca. 20 °C) while its upper layers are partially heated at different temperatures. We detect a remarkable temperature difference (ΔT) between the heated surface (x) and other areas of the device (y and z). Consequently, positive currents are detected by electrodes on either side of the top surface (x-y) and on the top and bottom (x-z) (Figure 2b). Additionally, the current curve is consistent with the surface temperature curve as a function of time (Figure 2c), which demonFigure 1. The morphologies and multicycle compressive properties of the RGOF. a,b) Cross- strates a precise and continuous thermal sectional FESEM images of the RGOF. c) Photographs of the free-standing and flexible paper- characterization ability. The above results like foam. d) Electrical resistance change when repeatedly compressed for over 19 cycles. The also indicate that ΔT guides the direction of WE and CE/RE stand for working electrode and counter/reference electrodes, respectively, and the temperature-activated carrier transport, F indicates the compression force. as illustrated in Figure 2a. The thermocurrent is around 9 µA for the RGOF sample possessing a 5.6 cm2 heating surface at a ΔT of about 30 °C, ture of RGOFs is a result of the freeze-casting induced in the above process. In freeze-casting, porous networks are formed which is measured with electrodes x and z. This value is only in frozen GO solution due to the ice template[28] and the netslightly lower than that measured with electrodes x and y (ca. 10 µA), which indicates the good carrier transmission capability work retains its connectivity when the ice is thawed and filof the RGOF in the thickness direction, which can be attribtered (Figure S1, Supporting Information). The RGOFs posuted to the graphene networks built during the freezing and sessing this nanostructure are free-standing and highly flexible filtering processes (Figure 1b). Additionally, the thermocurrent (Figure 1c), and more importantly, exhibit a sheet resistance as exhibits high stability and reversibility with successive on-off low as 1.5 ± 0.3 Ω sq−1, which is almost two orders of magheating (Figure S8, Supporting Information). The output of nitude lower than the lowest values ever reported from an the RGOF with a reverse connection to the external circuit is RGOF (ca. 100 Ω sq−1,[22] 91.2 Ω sq−1.[23] Additionally, the conalso measured, and it is found that the typical thermocurrent ductivity of these RGOFs (53 ± 3 S cm−1) is even better than remains roughly the same (ca. 9 µA) but the polarity is reversed that of the chemical vapor deposition-derived graphene foams (Figure S9, Supporting Information). This result confirms that (ca. 10 S cm−1).[23] This remarkably enhanced electrical property the measured electric signals are generated by the RGOF and may be attributed to highly conductive contacts between the not the measurement system. Figure 2d and S10 show the thergraphene sheets in our RGOF, as well as HI reduction-induced moelectric current as a function of ΔT. To measure the thermoiodine doping (Figure S2, S3, and Table S1, Supporting Inforelectric behaviors at ΔT < 0, the whole device is initially mainmation).[34,35] These results give the assurance of a good thertained at various temperatures and its surface is subsequently moelectric performance from these RGOF foams. cooled using ice. It is found that the thermoelectric current Remarkably, the RGOF can be compressed to at least 65% increases remarkably with increasing |ΔT| and, for a 5.6 cm2 strain and is still able to recover most of its original thickness rapidly when the loading is removed (see Figure S4 and heating surface, the current can reach around 550 µA at a ΔT of Movie S1, Supporting Information). Detailed hysteresis studies about 97.0 °C. Moreover, we find that the current increases lin(Figure S5, Supporting Information) show that it recovers at early when the heating surface is either in a free or a bending least at the speed of >180 mm min−1. Additionally, as shown state (Figure 2e). These results show that our RGOF is capable of sensing heat and cold, and of measuring the heated/cooled in Figure 1d and S6, the electrical resistance of the foam area under zero working voltage. decreases by 11% when the foam is compressed up to 68% strain under 2 kPa of pressure. The electrical properties can More interestingly, we find that the RGOF is capable of rapidly and completely recover to the original value within ca. discerning the human body from other objects (e.g., plastic, 0.05 s (Figure S7, Supporting Information), and the response metal, glass, rubber, etc.) under ambient conditions. As finger of the electrical resistance is found to be highly constant over pressure is applied, the output characteristics of the RGOF multiple cycles of compression. These results indicate that the undertake pronounced changes; while the read-out current
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COMMUNICATION Figure 2. Temperature sensing behaviors of the RGOF. a) Schematic of the device geometry illustrating the temperature-activated carrier transport through the temperature difference direction. b) Time-dependent thermocurrent measurements of the RGOF under on–off heat irradiation, where irradiation lasts 30 s (pink area). The thermocurrent is measured by electrodes x-y (red curve) and x-z (violet curve). c) Time-dependent thermocurrent and surface temperature curves of the RGOF under on-off heat irradiation. d) The ΔT-dependent thermocurrent curve of the sensor. The inset shows the magnified thermocurrent curve under the low ΔT range. e) Heating surface-dependent thermocurrent measurements of the sensor, which has been bent at different curvatures (r), as illustrated in the inset. f) Photographs show the sensor being touched by a human finger and other common objects. The current is measured by electrodes x-y (as illustrated in Figure 2a). Scale bar: 15 mm. The thermocurrent is measured by electrodes x-z in (c–e). The error bars are the the standard deviations calculated from repeated measurements.
remains unchanged when the RGOF is pressed using other common objects (Figure 2f and Movie S2–S4, Supporting Information). This is because body temperature is generally much higher than the temperature of our surroundings (Figure S11, Supporting Information), indicating that surface temperature is the most significant difference between a human body and an inanimate object. Therefore, owing to the unique temperatureactivated sensing mechanism, our RGOF sensor can recognize human touch. This unique feature can be the cornerstone for achieving the capability of social interaction or emotional communication required in next-generation robots.
Adv. Mater. 2014, DOI: 10.1002/adma.201401367
Encouraged by the efficient temperature-activated sensing behaviors of the RGOF, we subsequently demonstrate that the RGOF is able to sense human touch/pressure based on fingerheating effects. Figure 3a displays an infrared thermal image of the RGOF sensor. A finger touch on the sensor of no more than 1 s is able to generates a change in the maximum surface temperature, which can be seen in Figure 3a (area I; ΔT = 4.1 °C) where arrows indicate the carrier current due to temperature differences. Electric signals were thus detected by electrodes. Figure 3b and 3c show the electrical output performance of
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Figure 3. Characterization of the human touch response of the RGOF sensor. a) Infrared thermal image of the sensor surface, where area I is touched for 0.53 s, leading to a notable increase in surface temperature. Arrows indicate the carrier current. b) The current change upon placing and removing a finger (ca. 61 mm2) on the RGOF surface (areas I, II, and III), corresponding to a pressure of only 163 ± 12 Pa. c) The current change detected with a rapid and slow finger touch/remove cycle on area I, which reveals the high-speed and stable responsiveness of the RGOF sensor. Scale bar: 5 mm. d) Photograph (top panel) and corresponding infrared thermal image (middle panel) of the as-fabricated sensor pad. The bottom panel shows the bent sensor pad directly attached to a hand. Scale bar: 10 mm. e) Real-time current curves of the sensor pad during a finger touch/remove cycle on its surface. Curves shown in compartments A–I correspond to the touched areas A–I (see Figure 3d), respectively. Curves in each panel are measured by electrodes x1-y1 and x2-y2, as indicated in (E). Timescale bars: 5 s.
the RGOF sensor induced by finger touches. Pressures are measured to be 163 ± 12 Pa, which are very gentle touches that will not compress the foam. These finger-heating effects can have a direct impact upon the electrical behavior of RGOFs. The electrical signal displays high reversibility and stability and, more interestingly, according to Figure 3b, when the three different surface areas (I, II, and III) are touched the output current is about +200, 0, and –200 nA,
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respectively, which indicates that the RGOF sensors have the potential to determine the location of a finger touch by providing different types of electrical readout: positive, negative, or zero. We produce a proof-of-concept RGOF pressure sensor pad that is flexible and, except for the metal current collectors, an all-graphene device (Figure 3d). The device is fabricated by connecting an RGOF to four independent Cu foils
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COMMUNICATION Figure 4. Pressure sensing behaviors of the RGOF sensor. a) Infrared thermal images, real-time mean surface temperature curves and surfacetemperature distributions (corresponding to AR01 areas in infrared thermal images) of the RGOF (area A), demonstrating the thermal distributions on the surface of the sensor pad undergoing an ultra-light touch (165 Pa) or heavy pressure (49.0 kPa). Scale bars: 6 mm. b) Cross-sectional infrared thermal imaging macro photos, corresponding deep-dependent ΔT curves, and output current (measured by electrodes x2-y2) of the RGOF (area A), demonstrating the thermal distributions on the cross section of the sensor pad undergoing an ultra-light touch (165 Pa) or heavy pressure (49.0 kPa). The sensor pad in these measurements is placed on a Chinese coin (Figure S13, Supporting Information). The ΔT presents temperature changes in the thickness direction at the edge of the touched area. Insets show cross-sectional FESEM images of the RGOF under 165 Pa or 49.0 kPa. Scale bars: 2.5 µm. (c) Finger pressure-resolved current diagram on area A of the RGOF sensor-pad measured with electrodes x2-y2.
(x1, y1, x2, and y2) at its four sides whose metal collectors are connected to WEs and CEs/REs. This device can work as a 3 × 3 pixel pressure sensor pad that is capable of locating a fingertip touch. As illustrated in Figure 3d (middle panel) and 3e, the pressure sensor pad is partitioned into nine compartments, each using a ca. 64 mm2 area of the RGOF. This area limitation is due to the contact area of a typical fingertip (ca. 61 mm2, C. Hou’s index finger), and the current curves exhibited by each compartment displays the response of the device to a gentle touch (see Figure S12, Supporting Information). Based on the finger-heating effects, two ΔT-related signals are recorded as one result by the electrode lines x1-y1 and x2-y2 for each measurement. The device can give a total of nine different readout results that correspond to all possible locations in the 3 × 3 grid, thereby mapping the applied finger pressures with the sensor pad.
Adv. Mater. 2014, DOI: 10.1002/adma.201401367
An additional benefit of the elastic structure of the RGOF sensor pad is the ability to “map” the pressure levels by outputting the value-changeable thermocurrent. Typically, ultralight touches (163 ± 12 Pa) and heavy pressures (48.9 ± 0.3 kPa) are applied on the device. The temperature distribution on the surface (Figure 4a) and cross section (Figure 4b) of the device, together with the quantitative measurements, clearly show the diffusive characteristics of heat from the finger, which influences the thermocurrent. Figure 4a shows the obvious increase in the heating area (up to 99%) and decrease in the thermal diffusion velocity after finger removal when the RGOF is compressed under a relatively large pressure. Meanwhile, the rate of increase in the mean temperature difference also reaches 57% (from 2.4 to 3.3 °C), though it is worth noting that the finger contact area only increases by 3% in comparison. This indicates
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that the finger pressure rather than its contact area influences the finger heating effect to the greatest extent, wherein greater pressure by the finger induces a faster diffusion of the finger heat. The cross-sectional thermal imaging macro photos shown in Figure 4b further demonstrate the pressure-related thermal diffusion in the RGOF sensor pad in the thickness direction. Under relatively high pressure conditions, densification of the graphene networks is observed (FESEM images in Figure 4b), inducing a deeper diffusion of the finger heat due to the compact structure (Figure 4b). These results clearly show that a larger pressure induces more thermal energy in the RGOF. Finally, owing to the increasing thermal energy content, the readout thermocurrent measured from the compressed RGOF sensor pad increases to around 1500 nA, which is more than 7 times the value measured with a slight touch (ca. 200 nA). Figure 4c shows the finger pressure-resolved current diagram on area A of the RGOF sensor pad measured with electrodes x2-y2. The results indicate that the change in the finger pressure applied on the sensor-pad can be visually read from the output electrical signals. In addition to the fact that the all-graphene FTS in this study demonstrates desirable human touch-activated electrical behaviors, it also has a high sensitivity to pressure (p). The pressure from an ultra-light touch (163 ± 12 Pa) which does not initially compress the foam is defined as the base pressure for the devices, and any additional pressure is defined as the applied pressure (Δp). The sensitivity (S) can be defined as the slope of the trace in Figure S14 in the Supporting Information given by the relationship S = δ(ΔI/I0)/δΔp = (1/I0)δI/δΔp, where I is the current induced by applied pressure on the device and I0 is the device current with only a base pressure. The sensitivity of this device is found to be 15.2 kPa−1 when Δp is less than 300 Pa, which is much higher than the previously reported values obtained by an OFET sensor (8.4 kPa−1),[37] ferroelectret transistor sensor (10−3 kPa−1),[38] piezoelectric sensor (0.13 kPa−1),[39] capacitive sensor (0.55 kPa−1),[40] and resistance sensor (1.8 kPa−1).[17] This high pressure sensitivity relies mainly upon the high conductivity and the compressive (elastic) structure of this RGOF, and to the best of our knowledge, it has not been observed in other all-graphene thin-film materials. Recently, Park et al.[16] demonstrated a sensor array for the simultaneous sensing of pressure and temperature under an applied alternative current voltage with an amplitude of 5 V and a frequency of 0.3125 Hz. By comparison, our device achieves high sensitivity without applying any working voltage, which is one step closer to natural skin attributes. An unpowered FTS such as the one in this report is a passive device that relies solely upon the energy transmitted from the stimulus source, which is naturally available, and it is therefore more environmentally friendly than a self-powered sensor that uses its own electrical power from sources such as batteries, piezoelectric generator and solar cells. In conclusion, this report successfully demonstrates a highly conductive, flexible, and compressible all-graphene thin-film sensor that can sense heat and cold, measure the dimensions of the heated/cooled area, discern human touch from other pressures, and enable human touch locating and pressure level measuring under zero working voltage. The unique passive
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all-graphene FTS reported here may open up new doors for the development of electronic skin.
Experimental Section Synthesis of RGOFs: All the reagents were of analytical grade and used as obtained without further purification. The GO was synthesized through the Hummers’ method.[41] The GO foams were prepared by vacuum filtering a frozen GO solution using an Anodisc membrane (pore diameter: 0.2 µm), which were subsequently peeled off of the membrane and immersed into a 55% HI solution at room temperature for 1 h. The HI-reduced RGOFs were washed with deionized water several times. Fabrication of RGOF Sensor-Pad: The device was fabricated by connecting a RGOF to four independent Cu foils (x1, y1, x2, and y2) at its four sides. These metal collectors x1 and x2 were connected to CE/REs while y1 and y2 were connected to the WEs of the electrochemical workstations (CHI760D, Shanghai Chenhua Instruments) in measurements. Characterization and Measurements: The morphologies of the as-prepared samples were determined using a JSM-6700F FESEM, and the photographs were taken using a charge-coupled device (CCD) video camera (PowerShot G10, Canon). The electrical resistance of the as-prepared products was measured by the two-probe method using a Zahner electrochemical workstation (Zennium CIMPS-1), the applied voltage was 5 V. Compression experiments were conducted with a digital vernier caliper and an universal testing machine (INSTRON 5969). The electrical conductivity and sheet resistance were measured using the 4-point probe method (MCP-360, Mitsubishi Chemical Analytech Co. Ltd.) at room temperature. The thermoresponsivity measurements were carried out on an electrochemical workstation (CHI760D, Shanghai Chenhua Instruments). The RGOFs were integrated with small contact electrodes (Pt or Cu films) on the upper and lower surfaces, whose electrodes were connected to an electrochemical workstation. A solar simulator (Newport) equipped with a 300 W xenon lamp was used as a thermal source, whose light intensity was adjusted using an National Renewable Energy Laboratory-calibrated Si solar cell with a KG-1 filter for approximating one sun light intensity with Air Mass 1.5 Globlal (AM 1.5G). Ice and an electric heater were also used to control the temperature of the sensor’ surfaces. The integrated RGOF sensor pad was pressed by human fingers. The output current was measured under 0 V. The temperatures and infrared thermal images were recorded using an FLIR Thermo-Vision A40M infrared thermometer. The finger pressure (p) was calculated using the relationship p = N/S, where N is the load and, S is the area of the finger touch. A Mettler Toledo AL204 laboratory balance was placed under the device to collect the N data, and S was calculated by the grid method.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors gratefully acknowledge the financial support by the Natural Science Foundation of China (No. 51172042), the HighTech Research and Development Program of China (2012AA030309), the Specialized Research Fund for the Doctoral Program of Higher Education (20110075130001), the Science and Technology Commission of Shanghai Municipality (12nm0503900, 13JC1400200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Program for Changjiang Scholars and Innovation Research Team in University (TR2011079, IRT1221), the Fundamental Research Funds for the Central Universities, and
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Received: March 27, 2014 Revised: April 22, 2014 Published online:
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