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Qijun Sun, Do Hwan Kim, Sang Sik Park, Nae Yoon Lee, Yu Zhang, Jung Heon Lee, Kilwon Cho,* and Jeong Ho Cho* Electronic skin (e-skin) is a flexible circuit of sensors that can quantitatively detect a variety of stimuli and correlate these signals with a spatial mapping.[1–10] The development of a practical e-skin solution would be a significant milestone in the field of flexible health monitoring electronic devices. To conform to the human skin, e-skin devices should be flexible and stretchable. The devices should ideally be transparent so that they are visually imperceptible when worn. Beyond their mechanical and optical properties, the sensors need to, in practice, operate at low voltages so that the devices can be driven using portable thin-film batteries. Pressure sensor is a fundamental component of e-skin devices, and the preparation of highly sensitive large-area pressure sensors through low-cost fabrication procedures is critical for the development of e-skin.[11–17] Field-effect transistor (FET)type pressure sensor is an important class of these components and is expected to enable advanced sensing performance, including multi-parameter monitoring, a high sensitivity and resolution, and a low degree of signal crosstalk relative to capacitor-type sensors.[18,19] The current processes used to fabricate FET-type devices remain complicated, as they require a minimum of four steps: i) transistor fabrication, ii) device encapsulation, iii) via-hole opening, and iv) integration with pressure sensing components. The development of new FET Dr. Q. Sun, Prof. J. H. Cho SKKU Advanced Institute of Nanotechnology (SAINT) School of Chemical Engineering Sungkyunkwan University Suwon 440–746, Republic of Korea E-mail: [email protected] Dr. Q. Sun, Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology Pohang 790–784, Republic of Korea E-mail: [email protected] Prof. D. H. Kim, S. S. Park Department of Organic Materials and Fiber Engineering Soongsil University Seoul 156–743, Republic of Korea Prof. N. Y. Lee, Y. Zhang College of BioNano Technology Gachon University Seongnam 461–701, Republic of Korea Prof. J. H. Lee SKKU Advanced Institute of Nanotechnology (SAINT) School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440-746, Republic of Korea
DOI: 10.1002/adma.201400918
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Transparent, Low-Power Pressure Sensor Matrix Based on Coplanar-Gate Graphene Transistors
architectures that may be fabricated using simple processes is a priority in the field. Flexible transparent coplanar gate graphene field-effect transistors (GFETs) paired with an ion gel gate dielectric offer a feasible approach to prepare FET-type pressure sensors.[20] Ion gels, which consist of an ionic liquid and a gelating polymer, exhibit an extremely high capacitance,[21–24] in addition to their excellent mechanical flexibility and optical transparency. These qualities permit a low device operation voltage of less than 2 V. Moreover, the long-range polarization of ions in an ion gel can allow for an unconventional transistor geometry in which the gate electrode is coplanar with the source/drain electrodes. This arrangement can simplify device fabrication. We previously demonstrated that semi-metallic graphene could be used to form an active transistor channel and electrodes in an ion gel-gated high-performance GFET through a two-step assembly process.[20] This strategy can be further adapted toward constructing a pressure sensor matrix with pressure sensing units, FETs, and matrix layout lines integrated in a simplified process that dramatically reduced the complications associated with the four key steps summarized above. Herein, we firstly describe the successful development of a transparent GFET pressure sensor matrix (4 × 4 pixels) mounted on a plastic or rubber substrate for e-skin applications. The coplanar gate geometry of the GFETs based only on two materials (graphene and ion gel gate dielectric) was highly transparent, displayed a low power consumption, and could be fabricated through a simple process. The GFET pressure sensor was fabricated by laminating a top cover bearing a graphene square pattern onto a GFET backplane film bearing a pressuresensitive component. The application of pressure to the matrix induced contact between the square-type graphene on the top cover and the bottom zigzag (or interdigitated)-type graphene on the GFET backplane, thereby decreasing the resistance between the source and drain electrodes and leading to a higher transconductance. The devices exhibited excellent pressure sensor properties, including a high transparency ∼80% across the visible range, a low operating voltage of less than 2 V, a high pressure sensitivity of 0.12 kPa−1, and an excellent mechanical durability over 2500 cycles. The coplanar gate GFET pressure sensor matrix suggested by us provides a novel and simple route to achieving low-cost, flexible graphene electronics with high device performance. A schematic illustration of the ion gel-gated GFET with a coplanar configuration is shown in Figure 1a. The gate and drain electrodes of the GFETs were connected to the word line and bit line, respectively. A Au alignment marker was firstly patterned on the poly(ethylene terephthalate) (PET) film.
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Figure 1. (a) Schematic diagram of the pressure sensor based on coplanar gate GFETs with an ion gel gate dielectric. A zigzag-type source electrode was used. Gate and drain electrodes were connected to the word and bit lines, respectively. (b) Circuit design of the GFET pressure sensor matrix. (c) Integration of the top PET cover and the GFET matrix backplane. (d) Photographic images of a GFET pressure sensor matrix backplane, and optical microscopy image of one GFET pixel. The scale bar is 500 μm.
High-quality single-layer graphene[25–31] grown on a Cu foil (Figure S1) was transferred onto the PET film and then patterned by photolithography. The graphene pattern was divided into two component types: i) the matrix layout lines (word and bit lines), and ii) the coplanar gate GFETs with pressure sensing components (dashed blue frame). A UV-crosslinkable ion gel[32] gate dielectric was patterned across the gate electrode and a portion of the graphene strip. The UV-crosslinkable ion gel ink was composed of poly(ethylene glycol) diacrylate (PEGDA) monomers, the 2-hydroxy-2-methylpropiophenone (HOMPP) photo-initiator, and the 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([EMIM][TFSI]) ionic liquid (weight ratio of 7:3:90). The graphene strip segment in contact with the ion gel functioned as the transistor channel, whereas the remainder of the strip formed the source and drain electrodes. The channel length (L) and width (W) were 500 and 100 μm, respectively. The distance between the graphene gate and the channel was 150 μm. Two types of source electrodes with zigzag and interdigitated configurations were used to form the pressure sensing components. The line width and spacings of both source electrodes were designed to be 100 and 150 μm, respectively. We investigated the electrical performances of GFETs prepared with different numbers of zigzag periods, from 1 to 10, as shown in Figure S2. The GFETs prepared with 10 zigzag periods exhibited poor device performance and an extremely low ON/OFF current ratio of 1.6 due to the huge resistance in the zigzag period. Although the device performance was improved by decreasing the number of periods, the pressure sensing region area decreased dramatically, yielding a narrow pressure sensing range. We reasonably selected the
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GFET with 5 zigzag periods for subsequent pressure sensing tests. The GFET pressure sensor matrix for use in a flexible e-skin was fabricated according to a 4 × 4 GFETs matrix design, as shown in the circuit diagram in Figure 1b. Four devices were connected with word and bit lines to form a single group. Three other groups were positioned side by side. The matrix size defined by the Au alignment marker was 2.5 × 2.5 cm2, and each GFET was used as one pixel in the e-skin matrix. Finally, the top PET cover bearing 16 square-patterned graphene regions (2.5 × 2.5 mm2) was laminated onto the prepared GFET matrix backplane using 100 µm thick epoxy double sided tape (Figure 1c). Via holes in the top PET cover were prepared using a computer numerical control (CNC) milling machine. The procedure used to fabricate the GFET pressure sensor matrix is summarized in detail in Figure S3. The GFET pressure sensor matrix also exhibited good optical transparency. Figure S4 shows the optical transmittance over the visible range of the GFET pressure sensor matrix fabricated on a PET substrate, which revealed an overall transparency of 77%. We investigated the electrical performances of a coplanar gate GFET prepared with zigzag-type source electrodes, as shown in Figure 1a. Figure 2a shows the typical transfer characteristics of the coplanar gate GFET at five different drain voltages (VD). The ID values all increased as VG increased, both positively and negatively, indicating ambipolar charge transport in graphene upon ion gel gating. The GFET operated at a low gate voltage (VG) of less than 2 V due to the very large capacitance of the ion gel gate dielectric (7.9 μF/cm2). All 16 GFETs in the single matrix were functional, and the differences between
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COMMUNICATION Figure 2. (a) Transfer characteristics of the GFET at five different VD values. The inset shows the circuit diagram of a GFET pixel. (b) Transfer characteristics of the GFET pressure sensor under different pressures at VD = 0.3 V. (c) The conductance as a function of the applied pressure on the zigzag graphene electrode. (d) Sensitivity of the GFET pressure sensor.
the ID values of the 16 devices were less than 1 μA. The average carrier mobilities of 16 GFETs in one matrix were calculated to be 477 ± 73 cm2/V·s for holes and 166 ± 50 cm2/V·s for electrons, using methods reported previously,[20] as summarized in Figure S5. Importantly, ID increased gradually with VD, mainly due to the change in the potential difference between the source and drain electrodes. The pressure sensing characteristics of the GFET pixels were investigated using a force gauge driven by a step motor controller. A silicon wafer plate (10 mm2 in area) was adhered to the end of the force gauge pole to allow conformal contact between the square-patterned graphene on the top PET cover and the zigzag-type source electrode sensing components. Figure 2b showed the transfer characteristics at VD = 0.3 V under different applied pressures. In the absence of an applied pressure, the graphene square on the top PET cover did not contact the bottom zigzag sensing component. Charge carriers migrated only along the zigzag structure; thus, a relatively low ID of less than 2 μA was measured due to the high resistance of the zigzag structure. As the applied pressure was increased beyond 5 kPa, the contact area between the top graphene square and the bottom zigzag graphene increased gradually, as shown in the inset of Figure 2c. The increased contact area increased the conductance of the pressure sensing component (Figure 2c), which reduced the potential drop across this component. Therefore, a higher effective potential applied across the channel resulted in an increase in ID with increasing pressure. These pressuredependent transfer characteristics at a fixed VD were similar to the VD-dependent transfer characteristics. This observation
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was supported by the change in the potential drop across the graphene channel, which varied according to VD (Figure 2a) and the applied pressure (Figure 2b). Pressures beyond 40 kPa, however, did not increase ID because the contact area reached its maximum limitation as shown in Figure 2b (gray curve for 50 kPa and black curve for 60 kPa). After releasing the applied pressure, ID completely recovered its initial state. It should be noted that pressures below 5 kPa could not be detected due to the lack of contact between the top and bottom graphene components. The pressure sensing range may be further optimized by selecting an appropriate top cover film with a lower modulus and by decreasing the thickness of the epoxy spacer. Sensitivity (S), which is defined as (ΔI/I0)/P, where P is the applied pressure, I0 is the output current in the absence of pressure, and ΔI is I–I0, was calculated from the linear fit of the plot ΔI/I0 vs. P, as shown in Figure 2d. The sensitivity of our GFET pressure sensor to a 0.12 kPa−1 stimulus was much higher than the values reported previously using conductive rubber (0.05 kPa−1),[5,14,15] piezoelectric PVDF (0.02 kPa−1),[33] or ferroelectric materials (0.001 kPa−1);[34] however, the value was lower than the values of the pressure sensors based on microstructured polydimethyl siloxane (PDMS) (0.55 kPa−1)[35] or polyurethane–graphene sponges (0.26 kPa−1).[36] Although the GFET prepared with a zigzag electrode exhibited clear variations in ID under different pressures, it unfortunately did not display a significant OFF state upon release of the pressure. The zigzag structure was replaced with an interdigitated graphene structure in the GFET matrix, as shown in Figure 3a. The interdigitated structural design resembled
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Figure 3. (a) Schematic diagram of the GFET pressure sensor prepared with interdigitated source electrodes. The inset shows an optical microscopy image of a GFET pixel (scale bar 500 µm). (b) Transfer characteristics of the GFET prepared with interdigitated source electrodes under different pressures. (c) Real-time output current under different pressures at VD = 0.3 V and VG = −1 V. Pressures indicated by the arrows are given in units of kPa. (d) Durability test of the GFET pixels under the application and release of a 40 kPa pressure (VD = 0.3 V, VG = −1 V).
the zigzag structure with a 100 µm line width and a 150 μm spacing. One part of the interdigitated graphene was connected to the source electrode, and the other, including both the graphene channel and the drain electrode, was connected to the bit line. In this design, the interdigitated electrode formed an open circuit in the absence of an applied pressure. Application of a pressure exceeding 5 kPa to the top PET cover formed a current path. Figure 3b shows the transfer characteristics of GFETs prepared with interdigitated source electrodes under various pressures. A nano-ampere current was obtained without an external applied pressure, whereas a 15 micro-ampere current at VG = −1 V was obtained under a 40 kPa pressure. Real-time pressure sensing tests using the GFETs prepared with interdigitated electrodes were performed under different pressures, as shown in Figure 3c. The current values were compatible with the applied pressures. The ID response time upon application and release of the pressures was less than 0.5 s, calculated from the vertical rise and drop of ID. A cyclic durability test was conducted in which a 40 kPa pressure was repeatedly applied and released, as shown in Figure 3d. One cycle of pressure application and release was considered to be one cycle. Over 2500 cycles, a consistent 11 μA output current was maintained during pressure application, and the 1 nA current level was recovered upon release. These test results supported the durability and reliability of the e-skin matrix for practical applications. The pressures measured by the GFET pressure sensor matrix prepared with interdigitated source electrode were spatially mapped, as shown in Figure 4a. The polyacrylate letters ‘S’ (the first letter of ‘SKKU’) and ‘P’ (the first letter of
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‘POSTECH’) were positioned over the matrix, and a 40 kPa pressure was applied onto the letters. The word and bit line voltages were fixed at −1 and 0.3 V, respectively, for individual pixel addressing. The output currents of each pixel, including both the pressed and non-pressed pixels, were measured and plotted as a 2-D pressure color map. All pixels were found to be functional. A GFET pressure sensor matrix based on a zigzag-type source electrode was successfully fabricated on a PDMS rubber substrate for e-skin applications that require conformal contact and stretchability. Figure 4b shows the transfer characteristics of the GFET pressure sensor before and after applying a pressure. Under an applied pressure of 20 kPa, a reasonable increase in the output current at VG = −2 V was obtained from 1.3 to 1.8 μA. After releasing the pressure, the current level completely recovered to its initial current level. The insets in Figure 4b show that the GFET matrix on the rubber substrate could form conformal contact with a human hand and a table tennis ball. In summary, a low voltage transparent GFET-based pressure sensor matrix for e-skin application was prepared for the first time. Coplanar gate GFETs with a high-capacitance ion gel gate dielectric was used to form pressure sensors with a high transparency and a low power consumption. Importantly, the unconventional coplanar gate geometry and the use of graphene in the electrodes and semiconducting channels simplified the device fabrication process. The devices, which consisted of only two materials (graphene and the ion gel), displayed an excellent pressure sensor performance, with a high transparency of ∼80% over the visible range, a low operating voltage of less than
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COMMUNICATION Figure 4. (a) Spatial pressure map of the GFET pressure sensor matrix. The polyacrylate letters ‘S’ and ‘P’ were positioned over the matrix, and a 40 kPa pressure was applied. The output currents are plotted using 2-D color maps. (b) Pressure sensing characteristics of the GFET pressure sensor mounted on a PDMS rubber substrate. The inset shows that the GFET matrix made conformal contact with a human hand or a table tennis ball.
2 V, a high pressure sensitivity of 0.12 kPa−1, and an excellent mechanical durability, as demonstrated over 2500 pressure– release cycles. The pressure on the 4 × 4 GFETs matrix could be spatially mapped. We believe that the simple fabrication of a coplanar gate graphene pressure sensor matrix will advance further applications of graphene in transparent, flexible, and stretchable electronics.
numerical control (CNC) milling machine. Graphene was transferred onto the top PET cover and was patterned to form 16 squares (2.5 × 2.5 mm2). Finally, the top PET cover was laminated onto the GFET matrix backplane using 100 μm-thick epoxy double side tape (3M Scotch double sided tape Cat. 237). Device Characterization: A force gauge driven by a computer controlled stepping motor was connected with a probe station for the pressure sensing test. A semiconductor characterization system 4200 (SCS 4200) was used to characterize the electrical performance of the GFETs. All measurements were carried out under ambient condition.
Experimental Section Graphene Growth on a Cu Foil by CVD: A Cu foil (25 μm, Sigma Aldrich) was first cleaned with a piranha solution (mixture of H2SO4 and H2O2) for 15 min. The foil was then rinsed with DI water and dried under a nitrogen flow. Air was pumped after inserting the Cu foil into a quartz tube. Once the pressure in the quartz tube had reached 5 × 10−3 Torr, H2 (10 sccm) was introduced and the tube was heated to 1000 °C. The Cu foil was annealed under these conditions for 30 min, and 5 sccm CH4 was introduced to permit graphene growth under a continuous H2 (10 sccm) flow. After 30 min, the CH4 flow ceased and the tube was cooled to room temperature under a H2 flow. Thus, large-area graphene was obtained on a Cu foil (10 × 10 cm2). Fabrication of the GFET Pressure Sensor Matrix Backplane: The PET substrate was cleaned by ultrasonication in acetone, isopropanol, and DI water for 5 min each. An Au alignment marker was patterned on the substrate for alignment. A graphene monolayer was transferred onto the PET substrate, assisted by poly(methyl methacrylate) (PMMA). Photolithography was used to simultaneously pattern the matrix layout lines, the GFETs, and the sensing components in the same plane. The GFETs were prepared on polydimethylsiloxane (PDMS) by first patterning graphene on a Cu foil using photolithography. AZ 1512 was spin-coated onto the patterned graphene (in place of PMMA), and the structure was transferred onto a PDMS substrate. UV-crosslinkable ion gel dielectrics were patterned to cover both the gates and channels of the GFETs. The ion gel solution, a mixture of the 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([EMIM][TFSI]) ion liquid, the poly(ethylene glycol) diacrylate (PEGDA) monomer, and the 2-hydroxy2methylpropiophenone (HOMPP) photo-initiator (weight ratio of 90:7:3) was drop-cast. A square-patterned photomask was then placed over the ion gel layer, and the ion gel was exposed to UV light (100 mW/cm2 at 365 nm) for 5 s. Upon UV exposure, HOMPP generated radicals that could react with the acrylates in the PEG-DA monomers to initiate polymerization. The cross-linked ion gel was thereby photo-patterned only in the areas exposed to UV light, and the ion gel ink under the unexposed regions could be washed away using DI water. Fabrication of the Top PET Cover: Via holes in the top PET cover that matched the GFET matrix backplane were prepared using a computer
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Center for Advanced SoftElectronicsfunded by the Ministry of Science, ICT and Future Planning as Global FrontierProject (2013M3A6A5073177 and 2011-0031628) and Basic Science Research Program (2009-0083540) of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology, Korea. Received: February 27, 2014 Revised: April 1, 2014 Published online: May 20, 2014
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Qijun Sun, Do Hwan Kim, Sang Sik Park, Nae Yoon Lee, Yu Zhang, Jung Heon Lee, Kilwon Cho,* and Jeong Ho Cho* Electronic skin (e-skin) is a flexible circuit of sensors that can quantitatively detect a variety of stimuli and correlate these signals with a spatial mapping.[1–10] The development of a practical e-skin solution would be a significant milestone in the field of flexible health monitoring electronic devices. To conform to the human skin, e-skin devices should be flexible and stretchable. The devices should ideally be transparent so that they are visually imperceptible when worn. Beyond their mechanical and optical properties, the sensors need to, in practice, operate at low voltages so that the devices can be driven using portable thin-film batteries. Pressure sensor is a fundamental component of e-skin devices, and the preparation of highly sensitive large-area pressure sensors through low-cost fabrication procedures is critical for the development of e-skin.[11–17] Field-effect transistor (FET)type pressure sensor is an important class of these components and is expected to enable advanced sensing performance, including multi-parameter monitoring, a high sensitivity and resolution, and a low degree of signal crosstalk relative to capacitor-type sensors.[18,19] The current processes used to fabricate FET-type devices remain complicated, as they require a minimum of four steps: i) transistor fabrication, ii) device encapsulation, iii) via-hole opening, and iv) integration with pressure sensing components. The development of new FET Dr. Q. Sun, Prof. J. H. Cho SKKU Advanced Institute of Nanotechnology (SAINT) School of Chemical Engineering Sungkyunkwan University Suwon 440–746, Republic of Korea E-mail: [email protected] Dr. Q. Sun, Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology Pohang 790–784, Republic of Korea E-mail: [email protected] Prof. D. H. Kim, S. S. Park Department of Organic Materials and Fiber Engineering Soongsil University Seoul 156–743, Republic of Korea Prof. N. Y. Lee, Y. Zhang College of BioNano Technology Gachon University Seongnam 461–701, Republic of Korea Prof. J. H. Lee SKKU Advanced Institute of Nanotechnology (SAINT) School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440-746, Republic of Korea
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Transparent, Low-Power Pressure Sensor Matrix Based on Coplanar-Gate Graphene Transistors
architectures that may be fabricated using simple processes is a priority in the field. Flexible transparent coplanar gate graphene field-effect transistors (GFETs) paired with an ion gel gate dielectric offer a feasible approach to prepare FET-type pressure sensors.[20] Ion gels, which consist of an ionic liquid and a gelating polymer, exhibit an extremely high capacitance,[21–24] in addition to their excellent mechanical flexibility and optical transparency. These qualities permit a low device operation voltage of less than 2 V. Moreover, the long-range polarization of ions in an ion gel can allow for an unconventional transistor geometry in which the gate electrode is coplanar with the source/drain electrodes. This arrangement can simplify device fabrication. We previously demonstrated that semi-metallic graphene could be used to form an active transistor channel and electrodes in an ion gel-gated high-performance GFET through a two-step assembly process.[20] This strategy can be further adapted toward constructing a pressure sensor matrix with pressure sensing units, FETs, and matrix layout lines integrated in a simplified process that dramatically reduced the complications associated with the four key steps summarized above. Herein, we firstly describe the successful development of a transparent GFET pressure sensor matrix (4 × 4 pixels) mounted on a plastic or rubber substrate for e-skin applications. The coplanar gate geometry of the GFETs based only on two materials (graphene and ion gel gate dielectric) was highly transparent, displayed a low power consumption, and could be fabricated through a simple process. The GFET pressure sensor was fabricated by laminating a top cover bearing a graphene square pattern onto a GFET backplane film bearing a pressuresensitive component. The application of pressure to the matrix induced contact between the square-type graphene on the top cover and the bottom zigzag (or interdigitated)-type graphene on the GFET backplane, thereby decreasing the resistance between the source and drain electrodes and leading to a higher transconductance. The devices exhibited excellent pressure sensor properties, including a high transparency ∼80% across the visible range, a low operating voltage of less than 2 V, a high pressure sensitivity of 0.12 kPa−1, and an excellent mechanical durability over 2500 cycles. The coplanar gate GFET pressure sensor matrix suggested by us provides a novel and simple route to achieving low-cost, flexible graphene electronics with high device performance. A schematic illustration of the ion gel-gated GFET with a coplanar configuration is shown in Figure 1a. The gate and drain electrodes of the GFETs were connected to the word line and bit line, respectively. A Au alignment marker was firstly patterned on the poly(ethylene terephthalate) (PET) film.
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Figure 1. (a) Schematic diagram of the pressure sensor based on coplanar gate GFETs with an ion gel gate dielectric. A zigzag-type source electrode was used. Gate and drain electrodes were connected to the word and bit lines, respectively. (b) Circuit design of the GFET pressure sensor matrix. (c) Integration of the top PET cover and the GFET matrix backplane. (d) Photographic images of a GFET pressure sensor matrix backplane, and optical microscopy image of one GFET pixel. The scale bar is 500 μm.
High-quality single-layer graphene[25–31] grown on a Cu foil (Figure S1) was transferred onto the PET film and then patterned by photolithography. The graphene pattern was divided into two component types: i) the matrix layout lines (word and bit lines), and ii) the coplanar gate GFETs with pressure sensing components (dashed blue frame). A UV-crosslinkable ion gel[32] gate dielectric was patterned across the gate electrode and a portion of the graphene strip. The UV-crosslinkable ion gel ink was composed of poly(ethylene glycol) diacrylate (PEGDA) monomers, the 2-hydroxy-2-methylpropiophenone (HOMPP) photo-initiator, and the 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([EMIM][TFSI]) ionic liquid (weight ratio of 7:3:90). The graphene strip segment in contact with the ion gel functioned as the transistor channel, whereas the remainder of the strip formed the source and drain electrodes. The channel length (L) and width (W) were 500 and 100 μm, respectively. The distance between the graphene gate and the channel was 150 μm. Two types of source electrodes with zigzag and interdigitated configurations were used to form the pressure sensing components. The line width and spacings of both source electrodes were designed to be 100 and 150 μm, respectively. We investigated the electrical performances of GFETs prepared with different numbers of zigzag periods, from 1 to 10, as shown in Figure S2. The GFETs prepared with 10 zigzag periods exhibited poor device performance and an extremely low ON/OFF current ratio of 1.6 due to the huge resistance in the zigzag period. Although the device performance was improved by decreasing the number of periods, the pressure sensing region area decreased dramatically, yielding a narrow pressure sensing range. We reasonably selected the
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GFET with 5 zigzag periods for subsequent pressure sensing tests. The GFET pressure sensor matrix for use in a flexible e-skin was fabricated according to a 4 × 4 GFETs matrix design, as shown in the circuit diagram in Figure 1b. Four devices were connected with word and bit lines to form a single group. Three other groups were positioned side by side. The matrix size defined by the Au alignment marker was 2.5 × 2.5 cm2, and each GFET was used as one pixel in the e-skin matrix. Finally, the top PET cover bearing 16 square-patterned graphene regions (2.5 × 2.5 mm2) was laminated onto the prepared GFET matrix backplane using 100 µm thick epoxy double sided tape (Figure 1c). Via holes in the top PET cover were prepared using a computer numerical control (CNC) milling machine. The procedure used to fabricate the GFET pressure sensor matrix is summarized in detail in Figure S3. The GFET pressure sensor matrix also exhibited good optical transparency. Figure S4 shows the optical transmittance over the visible range of the GFET pressure sensor matrix fabricated on a PET substrate, which revealed an overall transparency of 77%. We investigated the electrical performances of a coplanar gate GFET prepared with zigzag-type source electrodes, as shown in Figure 1a. Figure 2a shows the typical transfer characteristics of the coplanar gate GFET at five different drain voltages (VD). The ID values all increased as VG increased, both positively and negatively, indicating ambipolar charge transport in graphene upon ion gel gating. The GFET operated at a low gate voltage (VG) of less than 2 V due to the very large capacitance of the ion gel gate dielectric (7.9 μF/cm2). All 16 GFETs in the single matrix were functional, and the differences between
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COMMUNICATION Figure 2. (a) Transfer characteristics of the GFET at five different VD values. The inset shows the circuit diagram of a GFET pixel. (b) Transfer characteristics of the GFET pressure sensor under different pressures at VD = 0.3 V. (c) The conductance as a function of the applied pressure on the zigzag graphene electrode. (d) Sensitivity of the GFET pressure sensor.
the ID values of the 16 devices were less than 1 μA. The average carrier mobilities of 16 GFETs in one matrix were calculated to be 477 ± 73 cm2/V·s for holes and 166 ± 50 cm2/V·s for electrons, using methods reported previously,[20] as summarized in Figure S5. Importantly, ID increased gradually with VD, mainly due to the change in the potential difference between the source and drain electrodes. The pressure sensing characteristics of the GFET pixels were investigated using a force gauge driven by a step motor controller. A silicon wafer plate (10 mm2 in area) was adhered to the end of the force gauge pole to allow conformal contact between the square-patterned graphene on the top PET cover and the zigzag-type source electrode sensing components. Figure 2b showed the transfer characteristics at VD = 0.3 V under different applied pressures. In the absence of an applied pressure, the graphene square on the top PET cover did not contact the bottom zigzag sensing component. Charge carriers migrated only along the zigzag structure; thus, a relatively low ID of less than 2 μA was measured due to the high resistance of the zigzag structure. As the applied pressure was increased beyond 5 kPa, the contact area between the top graphene square and the bottom zigzag graphene increased gradually, as shown in the inset of Figure 2c. The increased contact area increased the conductance of the pressure sensing component (Figure 2c), which reduced the potential drop across this component. Therefore, a higher effective potential applied across the channel resulted in an increase in ID with increasing pressure. These pressuredependent transfer characteristics at a fixed VD were similar to the VD-dependent transfer characteristics. This observation
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was supported by the change in the potential drop across the graphene channel, which varied according to VD (Figure 2a) and the applied pressure (Figure 2b). Pressures beyond 40 kPa, however, did not increase ID because the contact area reached its maximum limitation as shown in Figure 2b (gray curve for 50 kPa and black curve for 60 kPa). After releasing the applied pressure, ID completely recovered its initial state. It should be noted that pressures below 5 kPa could not be detected due to the lack of contact between the top and bottom graphene components. The pressure sensing range may be further optimized by selecting an appropriate top cover film with a lower modulus and by decreasing the thickness of the epoxy spacer. Sensitivity (S), which is defined as (ΔI/I0)/P, where P is the applied pressure, I0 is the output current in the absence of pressure, and ΔI is I–I0, was calculated from the linear fit of the plot ΔI/I0 vs. P, as shown in Figure 2d. The sensitivity of our GFET pressure sensor to a 0.12 kPa−1 stimulus was much higher than the values reported previously using conductive rubber (0.05 kPa−1),[5,14,15] piezoelectric PVDF (0.02 kPa−1),[33] or ferroelectric materials (0.001 kPa−1);[34] however, the value was lower than the values of the pressure sensors based on microstructured polydimethyl siloxane (PDMS) (0.55 kPa−1)[35] or polyurethane–graphene sponges (0.26 kPa−1).[36] Although the GFET prepared with a zigzag electrode exhibited clear variations in ID under different pressures, it unfortunately did not display a significant OFF state upon release of the pressure. The zigzag structure was replaced with an interdigitated graphene structure in the GFET matrix, as shown in Figure 3a. The interdigitated structural design resembled
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Figure 3. (a) Schematic diagram of the GFET pressure sensor prepared with interdigitated source electrodes. The inset shows an optical microscopy image of a GFET pixel (scale bar 500 µm). (b) Transfer characteristics of the GFET prepared with interdigitated source electrodes under different pressures. (c) Real-time output current under different pressures at VD = 0.3 V and VG = −1 V. Pressures indicated by the arrows are given in units of kPa. (d) Durability test of the GFET pixels under the application and release of a 40 kPa pressure (VD = 0.3 V, VG = −1 V).
the zigzag structure with a 100 µm line width and a 150 μm spacing. One part of the interdigitated graphene was connected to the source electrode, and the other, including both the graphene channel and the drain electrode, was connected to the bit line. In this design, the interdigitated electrode formed an open circuit in the absence of an applied pressure. Application of a pressure exceeding 5 kPa to the top PET cover formed a current path. Figure 3b shows the transfer characteristics of GFETs prepared with interdigitated source electrodes under various pressures. A nano-ampere current was obtained without an external applied pressure, whereas a 15 micro-ampere current at VG = −1 V was obtained under a 40 kPa pressure. Real-time pressure sensing tests using the GFETs prepared with interdigitated electrodes were performed under different pressures, as shown in Figure 3c. The current values were compatible with the applied pressures. The ID response time upon application and release of the pressures was less than 0.5 s, calculated from the vertical rise and drop of ID. A cyclic durability test was conducted in which a 40 kPa pressure was repeatedly applied and released, as shown in Figure 3d. One cycle of pressure application and release was considered to be one cycle. Over 2500 cycles, a consistent 11 μA output current was maintained during pressure application, and the 1 nA current level was recovered upon release. These test results supported the durability and reliability of the e-skin matrix for practical applications. The pressures measured by the GFET pressure sensor matrix prepared with interdigitated source electrode were spatially mapped, as shown in Figure 4a. The polyacrylate letters ‘S’ (the first letter of ‘SKKU’) and ‘P’ (the first letter of
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‘POSTECH’) were positioned over the matrix, and a 40 kPa pressure was applied onto the letters. The word and bit line voltages were fixed at −1 and 0.3 V, respectively, for individual pixel addressing. The output currents of each pixel, including both the pressed and non-pressed pixels, were measured and plotted as a 2-D pressure color map. All pixels were found to be functional. A GFET pressure sensor matrix based on a zigzag-type source electrode was successfully fabricated on a PDMS rubber substrate for e-skin applications that require conformal contact and stretchability. Figure 4b shows the transfer characteristics of the GFET pressure sensor before and after applying a pressure. Under an applied pressure of 20 kPa, a reasonable increase in the output current at VG = −2 V was obtained from 1.3 to 1.8 μA. After releasing the pressure, the current level completely recovered to its initial current level. The insets in Figure 4b show that the GFET matrix on the rubber substrate could form conformal contact with a human hand and a table tennis ball. In summary, a low voltage transparent GFET-based pressure sensor matrix for e-skin application was prepared for the first time. Coplanar gate GFETs with a high-capacitance ion gel gate dielectric was used to form pressure sensors with a high transparency and a low power consumption. Importantly, the unconventional coplanar gate geometry and the use of graphene in the electrodes and semiconducting channels simplified the device fabrication process. The devices, which consisted of only two materials (graphene and the ion gel), displayed an excellent pressure sensor performance, with a high transparency of ∼80% over the visible range, a low operating voltage of less than
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COMMUNICATION Figure 4. (a) Spatial pressure map of the GFET pressure sensor matrix. The polyacrylate letters ‘S’ and ‘P’ were positioned over the matrix, and a 40 kPa pressure was applied. The output currents are plotted using 2-D color maps. (b) Pressure sensing characteristics of the GFET pressure sensor mounted on a PDMS rubber substrate. The inset shows that the GFET matrix made conformal contact with a human hand or a table tennis ball.
2 V, a high pressure sensitivity of 0.12 kPa−1, and an excellent mechanical durability, as demonstrated over 2500 pressure– release cycles. The pressure on the 4 × 4 GFETs matrix could be spatially mapped. We believe that the simple fabrication of a coplanar gate graphene pressure sensor matrix will advance further applications of graphene in transparent, flexible, and stretchable electronics.
numerical control (CNC) milling machine. Graphene was transferred onto the top PET cover and was patterned to form 16 squares (2.5 × 2.5 mm2). Finally, the top PET cover was laminated onto the GFET matrix backplane using 100 μm-thick epoxy double side tape (3M Scotch double sided tape Cat. 237). Device Characterization: A force gauge driven by a computer controlled stepping motor was connected with a probe station for the pressure sensing test. A semiconductor characterization system 4200 (SCS 4200) was used to characterize the electrical performance of the GFETs. All measurements were carried out under ambient condition.
Experimental Section Graphene Growth on a Cu Foil by CVD: A Cu foil (25 μm, Sigma Aldrich) was first cleaned with a piranha solution (mixture of H2SO4 and H2O2) for 15 min. The foil was then rinsed with DI water and dried under a nitrogen flow. Air was pumped after inserting the Cu foil into a quartz tube. Once the pressure in the quartz tube had reached 5 × 10−3 Torr, H2 (10 sccm) was introduced and the tube was heated to 1000 °C. The Cu foil was annealed under these conditions for 30 min, and 5 sccm CH4 was introduced to permit graphene growth under a continuous H2 (10 sccm) flow. After 30 min, the CH4 flow ceased and the tube was cooled to room temperature under a H2 flow. Thus, large-area graphene was obtained on a Cu foil (10 × 10 cm2). Fabrication of the GFET Pressure Sensor Matrix Backplane: The PET substrate was cleaned by ultrasonication in acetone, isopropanol, and DI water for 5 min each. An Au alignment marker was patterned on the substrate for alignment. A graphene monolayer was transferred onto the PET substrate, assisted by poly(methyl methacrylate) (PMMA). Photolithography was used to simultaneously pattern the matrix layout lines, the GFETs, and the sensing components in the same plane. The GFETs were prepared on polydimethylsiloxane (PDMS) by first patterning graphene on a Cu foil using photolithography. AZ 1512 was spin-coated onto the patterned graphene (in place of PMMA), and the structure was transferred onto a PDMS substrate. UV-crosslinkable ion gel dielectrics were patterned to cover both the gates and channels of the GFETs. The ion gel solution, a mixture of the 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([EMIM][TFSI]) ion liquid, the poly(ethylene glycol) diacrylate (PEGDA) monomer, and the 2-hydroxy2methylpropiophenone (HOMPP) photo-initiator (weight ratio of 90:7:3) was drop-cast. A square-patterned photomask was then placed over the ion gel layer, and the ion gel was exposed to UV light (100 mW/cm2 at 365 nm) for 5 s. Upon UV exposure, HOMPP generated radicals that could react with the acrylates in the PEG-DA monomers to initiate polymerization. The cross-linked ion gel was thereby photo-patterned only in the areas exposed to UV light, and the ion gel ink under the unexposed regions could be washed away using DI water. Fabrication of the Top PET Cover: Via holes in the top PET cover that matched the GFET matrix backplane were prepared using a computer
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Center for Advanced SoftElectronicsfunded by the Ministry of Science, ICT and Future Planning as Global FrontierProject (2013M3A6A5073177 and 2011-0031628) and Basic Science Research Program (2009-0083540) of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology, Korea. Received: February 27, 2014 Revised: April 1, 2014 Published online: May 20, 2014
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